Hi Wei,
On Wed, Sep 01, 2021 at 11:12:58AM +0000, Wei Chen wrote:
Hi Akashi,
-----Original Message----- From: AKASHI Takahiro takahiro.akashi@linaro.org Sent: 2021年8月31日 14:18 To: Wei Chen Wei.Chen@arm.com Cc: Oleksandr Tyshchenko olekstysh@gmail.com; Stefano Stabellini sstabellini@kernel.org; Alex Benn??e alex.bennee@linaro.org; Kaly Xin Kaly.Xin@arm.com; Stratos Mailing List stratos-dev@op-lists.linaro.org; virtio-dev@lists.oasis-open.org; Arnd Bergmann arnd.bergmann@linaro.org; Viresh Kumar viresh.kumar@linaro.org; Stefano Stabellini stefano.stabellini@xilinx.com; stefanha@redhat.com; Jan Kiszka jan.kiszka@siemens.com; Carl van Schaik cvanscha@qti.qualcomm.com; pratikp@quicinc.com; Srivatsa Vaddagiri vatsa@codeaurora.org; Jean- Philippe Brucker jean-philippe@linaro.org; Mathieu Poirier mathieu.poirier@linaro.org; Oleksandr Tyshchenko Oleksandr_Tyshchenko@epam.com; Bertrand Marquis Bertrand.Marquis@arm.com; Artem Mygaiev Artem_Mygaiev@epam.com; Julien Grall julien@xen.org; Juergen Gross jgross@suse.com; Paul Durrant paul@xen.org; Xen Devel xen-devel@lists.xen.org Subject: Re: Enabling hypervisor agnosticism for VirtIO backends
Wei,
On Thu, Aug 26, 2021 at 12:10:19PM +0000, Wei Chen wrote:
Hi Akashi,
-----Original Message----- From: AKASHI Takahiro takahiro.akashi@linaro.org Sent: 2021年8月26日 17:41 To: Wei Chen Wei.Chen@arm.com Cc: Oleksandr Tyshchenko olekstysh@gmail.com; Stefano Stabellini sstabellini@kernel.org; Alex Benn??e alex.bennee@linaro.org; Kaly
Xin
Kaly.Xin@arm.com; Stratos Mailing List <stratos-dev@op-
lists.linaro.org>;
virtio-dev@lists.oasis-open.org; Arnd Bergmann
Viresh Kumar viresh.kumar@linaro.org; Stefano Stabellini stefano.stabellini@xilinx.com; stefanha@redhat.com; Jan Kiszka jan.kiszka@siemens.com; Carl van Schaik cvanscha@qti.qualcomm.com; pratikp@quicinc.com; Srivatsa Vaddagiri vatsa@codeaurora.org; Jean- Philippe Brucker jean-philippe@linaro.org; Mathieu Poirier mathieu.poirier@linaro.org; Oleksandr Tyshchenko Oleksandr_Tyshchenko@epam.com; Bertrand Marquis Bertrand.Marquis@arm.com; Artem Mygaiev Artem_Mygaiev@epam.com;
Julien
Grall julien@xen.org; Juergen Gross jgross@suse.com; Paul Durrant paul@xen.org; Xen Devel xen-devel@lists.xen.org Subject: Re: Enabling hypervisor agnosticism for VirtIO backends
Hi Wei,
On Fri, Aug 20, 2021 at 03:41:50PM +0900, AKASHI Takahiro wrote:
On Wed, Aug 18, 2021 at 08:35:51AM +0000, Wei Chen wrote:
Hi Akashi，
> -----Original Message----- > From: AKASHI Takahiro takahiro.akashi@linaro.org > Sent: 2021年8月18日 13:39 > To: Wei Chen Wei.Chen@arm.com > Cc: Oleksandr Tyshchenko olekstysh@gmail.com; Stefano
Stabellini
> sstabellini@kernel.org; Alex Benn??e alex.bennee@linaro.org;
Stratos
> Mailing List stratos-dev@op-lists.linaro.org; virtio-
dev@lists.oasis-
> open.org; Arnd Bergmann arnd.bergmann@linaro.org; Viresh Kumar > viresh.kumar@linaro.org; Stefano Stabellini > stefano.stabellini@xilinx.com; stefanha@redhat.com; Jan Kiszka > jan.kiszka@siemens.com; Carl van Schaik
> pratikp@quicinc.com; Srivatsa Vaddagiri vatsa@codeaurora.org;
Jean-
> Philippe Brucker jean-philippe@linaro.org; Mathieu Poirier > mathieu.poirier@linaro.org; Oleksandr Tyshchenko > Oleksandr_Tyshchenko@epam.com; Bertrand Marquis > Bertrand.Marquis@arm.com; Artem Mygaiev
Julien
> Grall julien@xen.org; Juergen Gross jgross@suse.com; Paul
Durrant
> paul@xen.org; Xen Devel xen-devel@lists.xen.org > Subject: Re: Enabling hypervisor agnosticism for VirtIO backends > > On Tue, Aug 17, 2021 at 08:39:09AM +0000, Wei Chen wrote: > > Hi Akashi, > > > > > -----Original Message----- > > > From: AKASHI Takahiro takahiro.akashi@linaro.org > > > Sent: 2021年8月17日 16:08 > > > To: Wei Chen Wei.Chen@arm.com > > > Cc: Oleksandr Tyshchenko olekstysh@gmail.com; Stefano
Stabellini
> > > sstabellini@kernel.org; Alex Benn??e
> Stratos > > > Mailing List stratos-dev@op-lists.linaro.org; virtio- > dev@lists.oasis- > > > open.org; Arnd Bergmann arnd.bergmann@linaro.org; Viresh
Kumar
> > > viresh.kumar@linaro.org; Stefano Stabellini > > > stefano.stabellini@xilinx.com; stefanha@redhat.com; Jan
Kiszka
> > > jan.kiszka@siemens.com; Carl van Schaik
> > > pratikp@quicinc.com; Srivatsa Vaddagiri
Jean-
> > > Philippe Brucker jean-philippe@linaro.org; Mathieu Poirier > > > mathieu.poirier@linaro.org; Oleksandr Tyshchenko > > > Oleksandr_Tyshchenko@epam.com; Bertrand Marquis > > > Bertrand.Marquis@arm.com; Artem Mygaiev
> Julien > > > Grall julien@xen.org; Juergen Gross jgross@suse.com;
Paul
Durrant
> > > paul@xen.org; Xen Devel xen-devel@lists.xen.org > > > Subject: Re: Enabling hypervisor agnosticism for VirtIO
backends
> > > > > > Hi Wei, Oleksandr, > > > > > > On Mon, Aug 16, 2021 at 10:04:03AM +0000, Wei Chen wrote: > > > > Hi All, > > > > > > > > Thanks for Stefano to link my kvmtool for Xen proposal
here.
> > > > This proposal is still discussing in Xen and KVM
communities.
> > > > The main work is to decouple the kvmtool from KVM and make > > > > other hypervisors can reuse the virtual device
implementations.
> > > > > > > > In this case, we need to introduce an intermediate
hypervisor
> > > > layer for VMM abstraction, Which is, I think it's very
close
> > > > to stratos' virtio hypervisor agnosticism work. > > > > > > # My proposal[1] comes from my own idea and doesn't always
represent
> > > # Linaro's view on this subject nor reflect Alex's concerns. > Nevertheless, > > > > > > Your idea and my proposal seem to share the same background. > > > Both have the similar goal and currently start with, at
first,
Xen
> > > and are based on kvm-tool. (Actually, my work is derived
from
> > > EPAM's virtio-disk, which is also based on kvm-tool.) > > > > > > In particular, the abstraction of hypervisor interfaces has
a
same
> > > set of interfaces (for your "struct vmm_impl" and my "RPC
interfaces").
> > > This is not co-incident as we both share the same origin as
I
said
> above. > > > And so we will also share the same issues. One of them is a
way
of
> > > "sharing/mapping FE's memory". There is some trade-off
between
> > > the portability and the performance impact. > > > So we can discuss the topic here in this ML, too. > > > (See Alex's original email, too). > > > > > Yes, I agree. > > > > > On the other hand, my approach aims to create a "single-
binary"
> solution > > > in which the same binary of BE vm could run on any
hypervisors.
> > > Somehow similar to your "proposal-#2" in [2], but in my
solution,
all
> > > the hypervisor-specific code would be put into another
entity
(VM),
> > > named "virtio-proxy" and the abstracted operations are
served
via RPC.
> > > (In this sense, BE is hypervisor-agnostic but might have OS > dependency.) > > > But I know that we need discuss if this is a requirement
even
> > > in Stratos project or not. (Maybe not) > > > > > > > Sorry, I haven't had time to finish reading your virtio-proxy
completely
> > (I will do it ASAP). But from your description, it seems we
need a
> > 3rd VM between FE and BE? My concern is that, if my assumption
is
right,
> > will it increase the latency in data transport path? Even if
we're
> > using some lightweight guest like RTOS or Unikernel, > > Yes, you're right. But I'm afraid that it is a matter of degree. > As far as we execute 'mapping' operations at every fetch of
payload,
> we will see latency issue (even in your case) and if we have
some
solution
> for it, we won't see it neither in my proposal :) >
Oleksandr has sent a proposal to Xen mailing list to reduce this
kind
of "mapping/unmapping" operations. So the latency caused by this
behavior
on Xen may eventually be eliminated, and Linux-KVM doesn't have
that
problem.
Obviously, I have not yet caught up there in the discussion. Which patch specifically?
Can you give me the link to the discussion or patch, please?
It's a RFC discussion. We have tested this RFC patch internally. https://lists.xenproject.org/archives/html/xen-devel/2021-
07/msg01532.html
I'm afraid that I miss something here, but I don't know why this proposed API will lead to eliminating 'mmap' in accessing the queued payload at every request?
This API give Xen device model (QEMU or kvmtool) the ability to map whole guest RAM in device model's address space. In this case, device model doesn't need dynamic hypercall to map/unmap payload memory. It can use a flat offset to access payload memory in its address space directly. Just Like KVM device model does now.
Thank you. Quickly, let me make sure one thing: This API itself doesn't do any mapping operations, right? So I suppose that virtio BE guest is responsible to 1) fetch the information about all the memory regions in FE, 2) call this API to allocate a big chunk of unused space in BE, 3) create grant/foreign mappings for FE onto this region(S) in the initialization/configuration of emulated virtio devices.
Is this the way this API is expected to be used? Does Xen already has an interface for (1)?
-Takahiro Akashi
Before this API, When device model to map whole guest memory, will severely consume the physical pages of Dom-0/Dom-D.
-Takahiro Akashi
Thanks, -Takahiro Akashi
-Takahiro Akashi
> > > Specifically speaking about kvm-tool, I have a concern about
its
> > > license term; Targeting different hypervisors and different
OSs
> > > (which I assume includes RTOS's), the resultant library
should
be
> > > license permissive and GPL for kvm-tool might be an issue. > > > Any thoughts? > > > > > > > Yes. If user want to implement a FreeBSD device model, but the
virtio
> > library is GPL. Then GPL would be a problem. If we have
another
good
> > candidate, I am open to it. > > I have some candidates, particularly for vq/vring, in my mind: > * Open-AMP, or > * corresponding Free-BSD code >
Interesting, I will look into them : )
Cheers, Wei Chen
> -Takahiro Akashi > > > > > -Takahiro Akashi > > > > > > > > > [1] https://op-lists.linaro.org/pipermail/stratos-dev/2021- > > > August/000548.html > > > [2] https://marc.info/?l=xen-devel&m=162373754705233&w=2 > > > > > > > > > > > > From: Oleksandr Tyshchenko olekstysh@gmail.com > > > > > Sent: 2021年8月14日 23:38 > > > > > To: AKASHI Takahiro takahiro.akashi@linaro.org;
Stefano
> Stabellini > > > sstabellini@kernel.org > > > > > Cc: Alex Benn??e alex.bennee@linaro.org; Stratos
Mailing
List
> > > stratos-dev@op-lists.linaro.org; virtio-dev@lists.oasis-
open.org;
> Arnd > > > Bergmann arnd.bergmann@linaro.org; Viresh Kumar > > > viresh.kumar@linaro.org; Stefano Stabellini > > > stefano.stabellini@xilinx.com; stefanha@redhat.com; Jan
Kiszka
> > > jan.kiszka@siemens.com; Carl van Schaik
> > > pratikp@quicinc.com; Srivatsa Vaddagiri
Jean-
> > > Philippe Brucker jean-philippe@linaro.org; Mathieu Poirier > > > mathieu.poirier@linaro.org; Wei Chen Wei.Chen@arm.com;
Oleksandr
> > > Tyshchenko Oleksandr_Tyshchenko@epam.com; Bertrand Marquis > > > Bertrand.Marquis@arm.com; Artem Mygaiev
> Julien > > > Grall julien@xen.org; Juergen Gross jgross@suse.com;
Paul
Durrant
> > > paul@xen.org; Xen Devel xen-devel@lists.xen.org > > > > > Subject: Re: Enabling hypervisor agnosticism for VirtIO
backends
> > > > > > > > > > Hello, all. > > > > > > > > > > Please see some comments below. And sorry for the
possible
format
> > > issues. > > > > > > > > > > > On Wed, Aug 11, 2021 at 9:27 AM AKASHI Takahiro > > > mailto:takahiro.akashi@linaro.org wrote: > > > > > > On Wed, Aug 04, 2021 at 12:20:01PM -0700, Stefano
Stabellini
> wrote: > > > > > > > CCing people working on Xen+VirtIO and IOREQs. Not
trimming
> the > > > original > > > > > > > email to let them read the full context. > > > > > > > > > > > > > > My comments below are related to a potential Xen > implementation, > > > not > > > > > > > because it is the only implementation that matters,
but
> because it > > > is > > > > > > > the one I know best. > > > > > > > > > > > > Please note that my proposal (and hence the working
prototype)[1]
> > > > > > is based on Xen's virtio implementation (i.e. IOREQ)
and
> > > particularly > > > > > > EPAM's virtio-disk application (backend server). > > > > > > It has been, I believe, well generalized but is still
a
bit
> biased > > > > > > toward this original design. > > > > > > > > > > > > So I hope you like my approach :) > > > > > > > > > > > > [1] https://op-lists.linaro.org/pipermail/stratos-
dev/2021-
> > > August/000546.html > > > > > > > > > > > > Let me take this opportunity to explain a bit more
about
my
> approach > > > below. > > > > > > > > > > > > > Also, please see this relevant email thread: > > > > > > > https://marc.info/?l=xen-devel&m=162373754705233&w=2 > > > > > > > > > > > > > > > > > > > > > On Wed, 4 Aug 2021, Alex Bennée wrote: > > > > > > > > Hi, > > > > > > > > > > > > > > > > One of the goals of Project Stratos is to enable
hypervisor
> > > agnostic > > > > > > > > backends so we can enable as much re-use of code
as
possible
> and > > > avoid > > > > > > > > repeating ourselves. This is the flip side of the
front end
> > > where > > > > > > > > multiple front-end implementations are required -
one
per OS,
> > > assuming > > > > > > > > you don't just want Linux guests. The resultant
guests
are
> > > trivially > > > > > > > > movable between hypervisors modulo any abstracted
paravirt
> type > > > > > > > > interfaces. > > > > > > > > > > > > > > > > In my original thumb nail sketch of a solution I
envisioned
> > > vhost-user > > > > > > > > daemons running in a broadly POSIX like
environment.
The
> > > interface to > > > > > > > > the daemon is fairly simple requiring only some
mapped
> memory > > > and some > > > > > > > > sort of signalling for events (on Linux this is
eventfd).
> The > > > idea was a > > > > > > > > stub binary would be responsible for any
hypervisor
specific
> > > setup and > > > > > > > > then launch a common binary to deal with the
actual
> virtqueue > > > requests > > > > > > > > themselves. > > > > > > > > > > > > > > > > Since that original sketch we've seen an expansion
in
the
> sort > > > of ways > > > > > > > > backends could be created. There is interest in > encapsulating > > > backends > > > > > > > > in RTOSes or unikernels for solutions like SCMI.
There
> interest > > > in Rust > > > > > > > > has prompted ideas of using the trait interface to
abstract
> > > differences > > > > > > > > away as well as the idea of bare-metal Rust
backends.
> > > > > > > > > > > > > > > > We have a card (STR-12) called "Hypercall
Standardisation"
> which > > > > > > > > calls for a description of the APIs needed from
the
> hypervisor > > > side to > > > > > > > > support VirtIO guests and their backends. However
we
are
> some > > > way off > > > > > > > > from that at the moment as I think we need to at
least
> > > demonstrate one > > > > > > > > portable backend before we start codifying
requirements. To
> that > > > end I > > > > > > > > want to think about what we need for a backend to
function.
> > > > > > > > > > > > > > > > Configuration > > > > > > > > ============= > > > > > > > > > > > > > > > > In the type-2 setup this is typically fairly
simple
because
> the > > > host > > > > > > > > system can orchestrate the various modules that
make
up the
> > > complete > > > > > > > > system. In the type-1 case (or even type-2 with
delegated
> > > service VMs) > > > > > > > > we need some sort of mechanism to inform the
backend
VM
> about > > > key > > > > > > > > details about the system: > > > > > > > > > > > > > > > > - where virt queue memory is in it's address
space
> > > > > > > > - how it's going to receive (interrupt) and
trigger
(kick)
> > > events > > > > > > > > - what (if any) resources the backend needs to
connect to
> > > > > > > > > > > > > > > > Obviously you can elide over configuration issues
by
having
> > > static > > > > > > > > configurations and baking the assumptions into
your
guest
> images > > > however > > > > > > > > this isn't scalable in the long term. The obvious
solution
> seems > > > to be > > > > > > > > extending a subset of Device Tree data to user
space
but
> perhaps > > > there > > > > > > > > are other approaches? > > > > > > > > > > > > > > > > Before any virtio transactions can take place the > appropriate > > > memory > > > > > > > > mappings need to be made between the FE guest and
the
BE
> guest. > > > > > > > > > > > > > > > Currently the whole of the FE guests address space
needs to
> be > > > visible > > > > > > > > to whatever is serving the virtio requests. I can
envision 3
> > > approaches: > > > > > > > > > > > > > > > > * BE guest boots with memory already mapped > > > > > > > > > > > > > > > > This would entail the guest OS knowing where in
it's
Guest
> > > Physical > > > > > > > > Address space is already taken up and avoiding
clashing. I
> > > would assume > > > > > > > > in this case you would want a standard interface
to
> userspace > > > to then > > > > > > > > make that address space visible to the backend
daemon.
> > > > > > > > > > > > Yet another way here is that we would have well known
"shared
> > > memory" between > > > > > > VMs. I think that Jailhouse's ivshmem gives us good
insights on
> this > > > matter > > > > > > and that it can even be an alternative for hypervisor-
agnostic
> > > solution. > > > > > > > > > > > > (Please note memory regions in ivshmem appear as a PCI
device
> and > > > can be > > > > > > mapped locally.) > > > > > > > > > > > > I want to add this shared memory aspect to my virtio-
proxy,
but
> > > > > > the resultant solution would eventually look similar
to
ivshmem.
> > > > > > > > > > > > > > * BE guests boots with a hypervisor handle to
memory
> > > > > > > > > > > > > > > > The BE guest is then free to map the FE's memory
to
where
> it > > > wants in > > > > > > > > the BE's guest physical address space. > > > > > > > > > > > > > > I cannot see how this could work for Xen. There is
no
"handle"
> to > > > give > > > > > > > to the backend if the backend is not running in dom0.
So
for
> Xen I > > > think > > > > > > > the memory has to be already mapped > > > > > > > > > > > > In Xen's IOREQ solution (virtio-blk), the following
information
> is > > > expected > > > > > > to be exposed to BE via Xenstore: > > > > > > (I know that this is a tentative approach though.) > > > > > > - the start address of configuration space > > > > > > - interrupt number > > > > > > - file path for backing storage > > > > > > - read-only flag > > > > > > And the BE server have to call a particular hypervisor
interface
> to > > > > > > map the configuration space. > > > > > > > > > > Yes, Xenstore was chosen as a simple way to pass
configuration
> info to > > > the backend running in a non-toolstack domain. > > > > > I remember, there was a wish to avoid using Xenstore in
Virtio
> backend > > > itself if possible, so for non-toolstack domain, this could
done
with
> > > adjusting devd (daemon that listens for devices and launches
backends)
> > > > > to read backend configuration from the Xenstore anyway
and
pass it
> to > > > the backend via command line arguments. > > > > > > > > > > > > > Yes, in current PoC code we're using xenstore to pass
device
> > > configuration. > > > > We also designed a static device configuration parse
method
for
> Dom0less > > > or > > > > other scenarios don't have xentool. yes, it's from device
model
> command > > > line > > > > or a config file. > > > > > > > > > But, if ... > > > > > > > > > > > > > > > > > In my approach (virtio-proxy), all those Xen (or
hypervisor)-
> > > specific > > > > > > stuffs are contained in virtio-proxy, yet another VM,
to
hide
> all > > > details. > > > > > > > > > > ... the solution how to overcome that is already found
and
proven
> to > > > work then even better. > > > > > > > > > > > > > > > > > > > > > # My point is that a "handle" is not mandatory for
executing
> mapping. > > > > > > > > > > > > > and the mapping probably done by the > > > > > > > toolstack (also see below.) Or we would have to
invent a
new
> Xen > > > > > > > hypervisor interface and Xen virtual machine
privileges
to
> allow > > > this > > > > > > > kind of mapping. > > > > > > > > > > > > > If we run the backend in Dom0 that we have no
problems
of
> course. > > > > > > > > > > > > One of difficulties on Xen that I found in my approach
is
that
> > > calling > > > > > > such hypervisor intefaces (registering IOREQ, mapping
memory) is
> > > only > > > > > > allowed on BE servers themselvies and so we will have
to
extend
> > > those > > > > > > interfaces. > > > > > > This, however, will raise some concern on security and
privilege
> > > distribution > > > > > > as Stefan suggested. > > > > > > > > > > We also faced policy related issues with Virtio backend
running in
> > > other than Dom0 domain in a "dummy" xsm mode. In our target
system we
> run > > > the backend in a driver > > > > > domain (we call it DomD) where the underlying H/W
resides.
We
> trust it, > > > so we wrote policy rules (to be used in "flask" xsm mode) to
provide
> it > > > with a little bit more privileges than a simple DomU had. > > > > > Now it is permitted to issue device-model, resource and
memory
> > > mappings, etc calls. > > > > > > > > > > > > > > > > > > > > > > > > > > > To activate the mapping will > > > > > > > > require some sort of hypercall to the hypervisor.
I
can see
> two > > > options > > > > > > > > at this point: > > > > > > > > > > > > > > > > - expose the handle to userspace for
daemon/helper
to
> trigger > > > the > > > > > > > > mapping via existing hypercall interfaces. If
using a
> helper > > > you > > > > > > > > would have a hypervisor specific one to avoid
the
daemon
> > > having to > > > > > > > > care too much about the details or push that
complexity
> into > > > a > > > > > > > > compile time option for the daemon which would
result in
> > > different > > > > > > > > binaries although a common source base. > > > > > > > > > > > > > > > > - expose a new kernel ABI to abstract the
hypercall
> > > differences away > > > > > > > > in the guest kernel. In this case the
userspace
would
> > > essentially > > > > > > > > ask for an abstract "map guest N memory to
userspace
> ptr" > > > and let > > > > > > > > the kernel deal with the different hypercall
interfaces.
> > > This of > > > > > > > > course assumes the majority of BE guests would
be
Linux
> > > kernels and > > > > > > > > leaves the bare-metal/unikernel approaches to
their own
> > > devices. > > > > > > > > > > > > > > > > Operation > > > > > > > > ========= > > > > > > > > > > > > > > > > The core of the operation of VirtIO is fairly
simple.
Once
> the > > > > > > > > vhost-user feature negotiation is done it's a case
of
> receiving > > > update > > > > > > > > events and parsing the resultant virt queue for
data.
The
> vhost- > > > user > > > > > > > > specification handles a bunch of setup before that
point,
> mostly > > > to > > > > > > > > detail where the virt queues are set up FD's for
memory and
> > > event > > > > > > > > communication. This is where the envisioned stub
process
> would > > > be > > > > > > > > responsible for getting the daemon up and ready to
run.
This
> is > > > > > > > > currently done inside a big VMM like QEMU but I
suspect a
> modern > > > > > > > > approach would be to use the rust-vmm vhost crate.
It
would
> then > > > either > > > > > > > > communicate with the kernel's abstracted ABI or be
re-
> targeted > > > as a > > > > > > > > build option for the various hypervisors. > > > > > > > > > > > > > > One thing I mentioned before to Alex is that Xen
doesn't
have
> VMMs > > > the > > > > > > > way they are typically envisioned and described in
other
> > > environments. > > > > > > > Instead, Xen has IOREQ servers. Each of them
connects
> > > independently to > > > > > > > Xen via the IOREQ interface. E.g. today multiple
QEMUs
could
> be > > > used as > > > > > > > emulators for a single Xen VM, each of them
connecting
to Xen
> > > > > > > independently via the IOREQ interface. > > > > > > > > > > > > > > The component responsible for starting a daemon
and/or
setting
> up > > > shared > > > > > > > interfaces is the toolstack: the xl command and the > libxl/libxc > > > > > > > libraries. > > > > > > > > > > > > I think that VM configuration management (or
orchestration
in
> > > Startos > > > > > > jargon?) is a subject to debate in parallel. > > > > > > Otherwise, is there any good assumption to avoid it
right
now?
> > > > > > > > > > > > > Oleksandr and others I CCed have been working on
ways
for the
> > > toolstack > > > > > > > to create virtio backends and setup memory mappings.
They
> might be > > > able > > > > > > > to provide more info on the subject. I do think we
miss
a way
> to > > > provide > > > > > > > the configuration to the backend and anything else
that
the
> > > backend > > > > > > > might require to start doing its job. > > > > > > > > > > Yes, some work has been done for the toolstack to handle
Virtio
> MMIO > > > devices in > > > > > general and Virtio block devices in particular. However,
it
has
> not > > > been upstreaned yet. > > > > > Updated patches on review now: > > > > > https://lore.kernel.org/xen-devel/1621626361-29076-1-
git-
send-
> email- > > > olekstysh@gmail.com/ > > > > > > > > > > There is an additional (also important) activity to
improve/fix
> > > foreign memory mapping on Arm which I am also involved in. > > > > > The foreign memory mapping is proposed to be used for
Virtio
> backends > > > (device emulators) if there is a need to run guest OS
completely
> > > unmodified. > > > > > Of course, the more secure way would be to use grant
memory
> mapping. > > > Brietly, the main difference between them is that with
foreign
mapping
> the > > > backend > > > > > can map any guest memory it wants to map, but with grant
mapping
> it is > > > allowed to map only what was previously granted by the
frontend.
> > > > > > > > > > So, there might be a problem if we want to pre-map some
guest
> memory > > > in advance or to cache mappings in the backend in order to
improve
> > > performance (because the mapping/unmapping guest pages every
request
> > > requires a lot of back and forth to Xen + P2M updates). In a
nutshell,
> > > currently, in order to map a guest page into the backend
address
space
> we > > > need to steal a real physical page from the backend domain.
So,
with
> the > > > said optimizations we might end up with no free memory in
the
backend
> > > domain (see XSA-300). And what we try to achieve is to not
waste
a
> real > > > domain memory at all by providing safe non-allocated-yet (so
unused)
> > > address space for the foreign (and grant) pages to be mapped
into,
> this > > > enabling work implies Xen and Linux (and likely DTB bindings)
changes.
> > > However, as it turned out, for this to work in a proper and
safe
way
> some > > > prereq work needs to be done. > > > > > You can find the related Xen discussion at: > > > > > https://lore.kernel.org/xen-devel/1627489110-25633-1-
git-
send-
> email- > > > olekstysh@gmail.com/ > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > One question is how to best handle notification
and
kicks.
> The > > > existing > > > > > > > > vhost-user framework uses eventfd to signal the
daemon
> (although > > > QEMU > > > > > > > > is quite capable of simulating them when you use
TCG).
Xen
> has > > > it's own > > > > > > > > IOREQ mechanism. However latency is an important
factor and
> > > having > > > > > > > > events go through the stub would add quite a lot. > > > > > > > > > > > > > > Yeah I think, regardless of anything else, we want
the
> backends to > > > > > > > connect directly to the Xen hypervisor. > > > > > > > > > > > > In my approach, > > > > > > a) BE -> FE: interrupts triggered by BE calling a
hypervisor
> > > interface > > > > > > via virtio-proxy > > > > > > b) FE -> BE: MMIO to config raises events (in event
channels),
> > > which is > > > > > > converted to a callback to BE via
virtio-
proxy
> > > > > > (Xen's event channel is internnally
implemented by
> > > interrupts.) > > > > > > > > > > > > I don't know what "connect directly" means here, but
sending
> > > interrupts > > > > > > to the opposite side would be best efficient. > > > > > > Ivshmem, I suppose, takes this approach by utilizing
PCI's
msi-x
> > > mechanism. > > > > > > > > > > Agree that MSI would be more efficient than SPI... > > > > > At the moment, in order to notify the frontend, the
backend
issues
> a > > > specific device-model call to query Xen to inject a
corresponding SPI
> to > > > the guest. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Could we consider the kernel internally converting
IOREQ
> > > messages from > > > > > > > > the Xen hypervisor to eventfd events? Would this
scale
with
> > > other kernel > > > > > > > > hypercall interfaces? > > > > > > > > > > > > > > > > So any thoughts on what directions are worth
experimenting
> with? > > > > > > > > > > > > > > One option we should consider is for each backend to
connect
> to > > > Xen via > > > > > > > the IOREQ interface. We could generalize the IOREQ
interface
> and > > > make it > > > > > > > hypervisor agnostic. The interface is really trivial
and
easy
> to > > > add. > > > > > > > > > > > > As I said above, my proposal does the same thing that
you
> mentioned > > > here :) > > > > > > The difference is that I do call hypervisor interfaces
via
> virtio- > > > proxy. > > > > > > > > > > > > > The only Xen-specific part is the notification
mechanism,
> which is > > > an > > > > > > > event channel. If we replaced the event channel with
something
> > > else the > > > > > > > interface would be generic. See: > > > > > > > https://gitlab.com/xen-project/xen/- > > > /blob/staging/xen/include/public/hvm/ioreq.h#L52 > > > > > > > > > > > > > > I don't think that translating IOREQs to eventfd in
the
kernel
> is > > > a > > > > > > > good idea: if feels like it would be extra
complexity
and that
> the > > > > > > > kernel shouldn't be involved as this is a backend-
hypervisor
> > > interface. > > > > > > > > > > > > Given that we may want to implement BE as a bare-metal > application > > > > > > as I did on Zephyr, I don't think that the translation
would not
> be > > > > > > a big issue, especially on RTOS's. > > > > > > It will be some kind of abstraction layer of interrupt
handling
> > > > > > (or nothing but a callback mechanism). > > > > > > > > > > > > > Also, eventfd is very Linux-centric and we are
trying to
> design an > > > > > > > interface that could work well for RTOSes too. If we
want to
> do > > > > > > > something different, both OS-agnostic and
hypervisor-
agnostic,
> > > perhaps > > > > > > > we could design a new interface. One that could be > implementable > > > in the > > > > > > > Xen hypervisor itself (like IOREQ) and of course any
other
> > > hypervisor > > > > > > > too. > > > > > > > > > > > > > > > > > > > > > There is also another problem. IOREQ is probably not
be
the
> only > > > > > > > interface needed. Have a look at > > > > > > > https://marc.info/?l=xen-devel&m=162373754705233&w=2.
Don't we
> > > also need > > > > > > > an interface for the backend to inject interrupts
into
the
> > > frontend? And > > > > > > > if the backend requires dynamic memory mappings of
frontend
> pages, > > > then > > > > > > > we would also need an interface to map/unmap domU
pages.
> > > > > > > > > > > > My proposal document might help here; All the
interfaces
> required > > > for > > > > > > virtio-proxy (or hypervisor-related interfaces) are
listed
as
> > > > > > RPC protocols :) > > > > > > > > > > > > > These interfaces are a lot more problematic than
IOREQ:
IOREQ
> is > > > tiny > > > > > > > and self-contained. It is easy to add anywhere. A
new
> interface to > > > > > > > inject interrupts or map pages is more difficult to
manage
> because > > > it > > > > > > > would require changes scattered across the various
emulators.
> > > > > > > > > > > > Exactly. I have no confident yet that my approach will
also
> apply > > > > > > to other hypervisors than Xen. > > > > > > Technically, yes, but whether people can accept it or
not
is a
> > > different > > > > > > matter. > > > > > > > > > > > > Thanks, > > > > > > -Takahiro Akashi > > > > > > > > > > > > > > > > > > > > -- > > > > > Regards, > > > > > > > > > > Oleksandr Tyshchenko > > > > IMPORTANT NOTICE: The contents of this email and any
attachments are
> > > confidential and may also be privileged. If you are not the
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> > > recipient, please notify the sender immediately and do not
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> the > > > information in any medium. Thank you. > > IMPORTANT NOTICE: The contents of this email and any
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> recipient, please notify the sender immediately and do not
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> contents to any other person, use it for any purpose, or store
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> information in any medium. Thank you. IMPORTANT NOTICE: The contents of this email and any attachments
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confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose
the
contents to any other person, use it for any purpose, or store or copy
the
information in any medium. Thank you.
IMPORTANT NOTICE: The contents of this email and any attachments are
confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.