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

From: Alex Williamson
Subject: Re: Out-of-Process Device Emulation session at KVM Forum 2020
Date: Thu, 29 Oct 2020 09:46:03 -0600

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
> >>>
> >>> management:
> >>>       * 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
> >>>
> >>> migration:
> >>>       * 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.

> 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

> > 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.

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


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