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

[Hanna Czenczek <hreitz@redhat.com>]

(By the way, thanks for the explanations :))

On 05.05.23 11:03, Hanna Czenczek wrote:
> On 04.05.23 23:14, Stefan Hajnoczi wrote:

[...]

>> I think it's better to change QEMU's vhost code
>> to leave stateful devices suspended (but not reset) across
>> vhost_dev_stop() -> vhost_dev_start(), maybe by introducing
>> vhost_dev_suspend() and vhost_dev_resume(). Have you thought about
>> this aspect?
>
> Yes and no; I mean, I haven’t in detail, but I thought this is what’s 
> meant by suspending instead of resetting when the VM is stopped.

So, now looking at vhost_dev_stop(), one problem I can see is that 
depending on the back-end, different operations it does will do 
different things.

It tries to stop the whole device via vhost_ops->vhost_dev_start(), 
which for vDPA will suspend the device, but for vhost-user will reset it 
(if F_STATUS is there).

It disables all vrings, which doesn’t mean stopping, but may be 
necessary, too.  (I haven’t yet really understood the use of disabled 
vrings, I heard that virtio-net would have a need for it.)

It then also stops all vrings, though, so that’s OK.  And because this 
will always do GET_VRING_BASE, this is actually always the same 
regardless of transport.

Finally (for this purpose), it resets the device status via 
vhost_ops->vhost_reset_status().  This is only implemented on vDPA, and 
this is what resets the device there.


So vhost-user resets the device in .vhost_dev_start, but vDPA only does 
so in .vhost_reset_status.  It would seem better to me if vhost-user 
would also reset the device only in .vhost_reset_status, not in 
.vhost_dev_start.  .vhost_dev_start seems precisely like the place to 
run SUSPEND/RESUME.

Another question I have (but this is basically what I wrote in my last 
email) is why we even call .vhost_reset_status here.  If the device 
and/or all of the vrings are already stopped, why do we need to reset 
it?  Naïvely, I had assumed we only really need to reset the device if 
the guest changes, so that a new guest driver sees a freshly initialized 
device.

Hanna