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Re: Virtio in Xen on Arm (based on IOREQ concept)

On 20.07.20 12:17, Roger Pau Monné wrote:

Hello Roger

On Fri, Jul 17, 2020 at 09:34:14PM +0300, Oleksandr wrote:
On 17.07.20 18:00, Roger Pau Monné wrote:
On Fri, Jul 17, 2020 at 05:11:02PM +0300, Oleksandr Tyshchenko wrote:
some implementation to forward guest MMIO access to a device model. And as
turned out the Xen on x86 contains most of the pieces to be able to use that
transport (via existing IOREQ concept). Julien has already done a big amount
of work in his PoC (xen/arm: Add support for Guest IO forwarding to a
device emulator).
Using that code as a base we managed to create a completely functional PoC
with DomU
running on virtio block device instead of a traditional Xen PV driver
modifications to DomU Linux. Our work is mostly about rebasing Julien's
code on the actual
codebase (Xen 4.14-rc4), various tweeks to be able to run emulator
(virtio-disk backend)
in other than Dom0 domain (in our system we have thin Dom0 and keep all
in driver domain),
How do you handle this use-case? Are you using grants in the VirtIO
ring, or rather allowing the driver domain to map all the guest memory
and then placing gfn on the ring like it's commonly done with VirtIO?
Second option. Xen grants are not used at all as well as event channel and
Xenbus. That allows us to have guest

*unmodified* which one of the main goals. Yes, this may sound (or even
sounds) non-secure, but backend which runs in driver domain is allowed to
map all guest memory.
Supporting unmodified guests is certainly a fine goal, but I don't
think it's incompatible with also trying to expand the spec in
parallel in order to support grants in a negotiated way (see below).

That way you could (long term) regain some of the lost security.

Do you have any plans to try to upstream a modification to the VirtIO
spec so that grants (ie: abstract references to memory addresses) can
be used on the VirtIO ring?
But VirtIO spec hasn't been modified as well as VirtIO infrastructure in the
guest. Nothing to upsteam)
OK, so there's no intention to add grants (or a similar interface) to
the spec?

I understand that you want to support unmodified VirtIO frontends, but
I also think that long term frontends could negotiate with backends on
the usage of grants in the shared ring, like any other VirtIO feature
negotiated between the frontend and the backend.

This of course needs to be on the spec first before we can start
implementing it, and hence my question whether a modification to the
spec in order to add grants has been considered.

It's fine to say that you don't have any plans in this regard.
Adding grants (or a similar interface) to the spec hasn't been considered so far.

But I understand and completely agree that some solution should be found in order not to reduce security.

misc fixes for our use-cases and tool support for the
Unfortunately, Julien doesn’t have much time to allocate on the work
so we would like to step in and continue.

*A few word about the Xen code:*
You can find the whole Xen series at [5]. The patches are in RFC state
some actions in the series should be reconsidered and implemented properly.
Before submitting the final code for the review the first IOREQ patch
(which is quite
big) will be split into x86, Arm and common parts. Please note, x86 part
even build-tested so far and could be broken with that series. Also the
series probably
wants splitting into adding IOREQ on Arm (should be focused first) and
tools support
for the virtio-disk (which is going to be the first Virtio driver)
configuration before going
into the mailing list.
Sending first a patch series to enable IOREQs on Arm seems perfectly
fine, and it doesn't have to come with the VirtIO backend. In fact I
would recommend that you send that ASAP, so that you don't spend time
working on the backend that would likely need to be modified
according to the review received on the IOREQ series.
Completely agree with you, I will send it after splitting IOREQ patch and
performing some cleanup.

However, it is going to take some time to make it properly taking into the

that personally I won't be able to test on x86.
We have gitlab and the osstest CI loop (plus all the reviewers) so we
should be able to spot any regressions. Build testing on x86 would be
nice so that you don't need to resend to fix build issues.

Of course, before sending series to ML I will definitely perform a build test

on x86.

What I would like to add here, the IOREQ feature on Arm could be used not
for implementing Virtio, but for other use-cases which require some
emulator entity
outside Xen such as custom PCI emulator (non-ECAM compatible) for example.

*A few word about the backend(s):*
One of the main problems with Virtio in Xen on Arm is the absence of
“ready-to-use” and “out-of-Qemu” Virtio backends (I least am not aware of).
We managed to create virtio-disk backend based on demu [3] and kvmtool [4]
that series. It is worth mentioning that although Xenbus/Xenstore is not
to be used with native Virtio, that interface was chosen to just pass
configuration from toolstack
to the backend and notify it about creating/destroying Guest domain (I
think it is
I would prefer if a single instance was launched to handle each
backend, and that the configuration was passed on the command line.
Killing the user-space backend from the toolstack is fine I think,
there's no need to notify the backend using xenstore or any other
out-of-band methods.

xenstore has proven to be a bottleneck in terms of performance, and it
would be better if we can avoid using it when possible, specially here
that you have to do this from scratch anyway.
Let me elaborate a bit more on this.

In current backend implementation, the Xenstore is *not* used for
communication between backend (VirtIO device) and frontend (VirtIO driver),
frontend knows nothing about it.

Xenstore was chosen as an interface in order to be able to pass
configuration from toolstack in Dom0 to backend which may reside in other
than Dom0 domain (DomD in our case),
There's 'xl devd' which can be used on the driver domain to spawn
backends, maybe you could add the logic there so that 'xl devd' calls
the backend executable with the required command line parameters, so
that the backend itself doesn't need to interact with xenstore in any

That way in the future we could use something else instead of
xenstore, like Argo for instance in order to pass the backend data
from the control domain to the driver domain.

also looking into the Xenstore entries backend always knows when the
intended guest is been created/destroyed.
xl devd should also do the killing of backends anyway when a domain is
destroyed, or else malfunctioning user-space backends could keep
running after the domain they are serving is destroyed.

I may mistake, but I don't think we can avoid using Xenstore (or other
interface provided by toolstack) for the several reasons.

Besides a virtio-disk configuration (a disk to be assigned to the guest, R/O
mode, etc), for each virtio-mmio device instance

a pair (mmio range + IRQ) are allocated by toolstack at the guest
construction time and inserted into virtio-mmio device tree node

in the guest device tree. And for the backend to properly operate these
variable parameters are also passed to the backend via Xenstore.
I think you could pass all these parameters as command line arguments
to the backend?

The other reasons are:

1. Automation. With current backend implementation we don't need to pause
guest right after creating it, then go to the driver domain and spawn
backend and

after that go back to the dom0 and unpause the guest.
xl devd should be capable of handling this for you on the driver

2. Ability to detect when guest with involved frontend has gone away and
properly release resource (guest destroy/reboot).

3. Ability to (re)connect to the newly created guest with involved frontend
(guest create/reboot).

4. What is more that having Xenstore support the backend is able to detect
the dom_id it runs into and the guest dom_id, there is no need pass them via
command line.

I will be happy to explain in details after publishing backend code).
As I'm not the one doing the work I certainly won't stop you from
using xenstore on the backend. I would certainly prefer if the backend
gets all the information it needs from the command line so that the
configuration data is completely agnostic to the transport layer used
to convey it.

Thanks, Roger.

Thank you for pointing another possible way. I feel I need to investigate what is the "xl devd" (+ Argo?) and how it works. If it is able to provide backend with

the support/information it needs and xenstore is not welcome then I would be absolutely ok to consider using other solution.

I propose to get back to that discussion after I prepare and send out the proper IOREQ series.


Oleksandr Tyshchenko



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