Re: [PATCH 0/4] vhost-user-fs: Internal migration

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, May 05, 2023 at 02:51:55PM +0200, Hanna Czenczek wrote:
> On 05.05.23 11:53, Eugenio Perez Martin wrote:
> > On Fri, May 5, 2023 at 11:03 AM Hanna Czenczek <hreitz@redhat.com> wrote:
> > > On 04.05.23 23:14, Stefan Hajnoczi wrote:
> > > > On Thu, 4 May 2023 at 13:39, Hanna Czenczek <hreitz@redhat.com> wrote:
> 
> [...]
> 
> > > > All state is lost and the Device Initialization process
> > > > must be followed to make the device operational again.
> > > > 
> > > > Existing vhost-user backends don't implement SET_STATUS 0 (it's new).
> > > > 
> > > > It's messy and not your fault. I think QEMU should solve this by
> > > > treating stateful devices differently from non-stateful devices. That
> > > > way existing vhost-user backends continue to work and new stateful
> > > > devices can also be supported.
> > > It’s my understanding that SET_STATUS 0/RESET_DEVICE is problematic for
> > > stateful devices.  In a previous email, you wrote that these should
> > > implement SUSPEND+RESUME so qemu can use those instead.  But those are
> > > separate things, so I assume we just use SET_STATUS 0 when stopping the
> > > VM because this happens to also stop processing vrings as a side effect?
> > > 
> > > I.e. I understand “treating stateful devices differently” to mean that
> > > qemu should use SUSPEND+RESUME instead of SET_STATUS 0 when the back-end
> > > supports it, and stateful back-ends should support it.
> > > 
> > Honestly I cannot think of any use case where the vhost-user backend
> > did not ignore set_status(0) and had to retrieve vq states. So maybe
> > we can totally remove that call from qemu?
> 
> I don’t know so I can’t really say; but I don’t quite understand why qemu
> would reset a device at any point but perhaps VM reset (and even then I’d
> expect the post-reset guest to just reset the device on boot by itself,
> too).

DPDK stores the Device Status field value and uses it later:
https://github.com/DPDK/dpdk/blob/main/lib/vhost/vhost_user.c#L2791

While DPDK performs no immediate action upon SET_STATUS 0, omitting the
message will change the behavior of other DPDK code like
virtio_is_ready().

Changing the semantics of the vhost-user protocol in a way that's not
backwards compatible is something we should avoid unless there is no
other way.

The fundamental problem is that QEMU's vhost code is designed to reset
vhost devices because it assumes they are stateless. If an F_SUSPEND
protocol feature bit is added, then it becomes possible to detect new
backends and suspend/resume them rather than reset them.

That's the solution that I favor because it's backwards compatible and
the same model can be applied to stateful vDPA devices in the future.

Stefan

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