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Re: Out-of-Process Device Emulation session at KVM Forum 2020

From: Jason Wang
Subject: Re: Out-of-Process Device Emulation session at KVM Forum 2020
Date: Fri, 30 Oct 2020 09:11:23 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 2020/10/29 下午11:46, Alex Williamson wrote:
On Thu, 29 Oct 2020 23:09:33 +0800
Jason Wang <jasowang@redhat.com> wrote:

On 2020/10/29 下午10:31, Alex Williamson wrote:
On Thu, 29 Oct 2020 21:02:05 +0800
Jason Wang <jasowang@redhat.com> wrote:
On 2020/10/29 下午8:08, Stefan Hajnoczi wrote:
Here are notes from the session:

protocol stability:
       * vhost-user already exists for existing third-party applications
       * vfio-user is more general but will take more time to develop
       * libvfio-user can be provided to allow device implementations

       * Should QEMU launch device emulation processes?
           * Nicer user experience
           * Technical blockers: forking, hotplug, security is hard once
QEMU has started running
           * Probably requires a new process model with a long-running
QEMU management process proxying QMP requests to the emulator process

       * dbus-vmstate
       * VFIO live migration ioctls
           * Source device can continue if migration fails
           * Opaque blobs are transferred to destination, destination can
fail migration if it decides the blobs are incompatible
I'm not sure this can work:

1) Reading something that is opaque to userspace is probably a hint of
bad uAPI design
2) Did qemu even try to migrate opaque blobs before? It's probably a bad
design of migration protocol as well.

It looks to me have a migration driver in qemu that can clearly define
each byte in the migration stream is a better approach.
Any time during the previous two years of development might have been a
more appropriate time to express your doubts.

Somehow I did that in this series[1]. But the main issue is still there.
That series is related to a migration compatibility interface, not the
migration data itself.

They are not independent. The compatibility interface design depends on the migration data design. I ask the uAPI issue in that thread but without any response.

Is this legal to have a uAPI that turns out to be opaque to userspace?
(VFIO seems to be the first). If it's not,  the only choice is to do
that in Qemu.
So you're suggesting that any time the kernel is passing through opaque
data that gets interpreted by some entity elsewhere, potentially with
proprietary code, that we're in legal jeopardy?  VFIO is certainly not
the first to do that (storage and network devices come to mind).
Devices are essentially opaque data themselves, vfio provides access to
(ex.) BARs, but the interpretation of what resides in that BAR is device
specific.  Sometimes it's defined in a public datasheet, sometimes not.
Suggesting that we can't move opaque data through a uAPI seems rather

No, I think we are talking about different things. What I meant is the data carried via uAPI should not opaque userspace. What you said here is a good example for this actually. When you expose BAR to userspace, there should be driver that knows the semantics of BAR running in the userspace, so it's not opaque to userspace.

Note that we're not talking about vDPA devices here, we're talking
about arbitrary devices with arbitrary state.  Some degree of migration
support for assigned devices can be implemented in QEMU, Alex Graf
proved this several years ago with i40evf.  Years later, we don't have
any vendors proposing device specific migration code for QEMU.

Yes but it's not necessarily VFIO as well.
I don't know what this means.

I meant we can't not assume VFIO is the only uAPI that will be used by Qemu.

Clearly we're also trying to account for proprietary devices where even
for suspend/resume support, proprietary drivers may be required for
manipulating that internal state.  When we move device emulation
outside of QEMU, whether in kernel or to other userspace processes,
does it still make sense to require code in QEMU to support
interpretation of that device for migration purposes?

Well, we could extend Qemu to support property module (or have we
supported that now?). And then it can talk to property drivers via
either VFIO or vendor specific uAPI.
Yikes, I thought out-of-process devices was exactly the compromise
being developed to avoid QEMU supporting proprietary modules and ad-hoc
vendor specific uAPIs.

We can't even prevent this in kernel, so I don't see how possible we can make it for Qemu.

I think you're actually questioning even the
premise of developing a standardized API for out-of-process devices
here.  Thanks,

Actually not, it's just question in my mind when looking at VFIO migration compatibility patches, since vfio-user is being proposed, it's a good time to revisit them.



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