Re: [virtio-comment] [PATCH RFC] virtio: introduce VIRTIO_F_DEVICE_STOP

[Jason Wang <jasowang@redhat.com>]

On 2021/1/12 äå6:54, Michael S. Tsirkin wrote:
> On Tue, Dec 29, 2020 at 04:04:27AM -0500, Jason Wang wrote:
> > > > > > However wouldn't it be possible for device to just cancel
> > > > > > processing available buffers even if it started processing
> > > > > > them? Some buffers could be in indeterminate state
> > > > > > (e.g. we might not have a way to know how much data did
> > > > > > device have time to write into a buffer).
> > > > > Maybe Jason can answer this one.
> > > >
> > > > For networking device, it should be possible (packet could be dropped or
> > > > duplicated). But I'm not sure it works for other device. E.g for the block
> > > > devices that needs to communicate with a remote backend. Waat's more
> > > > important, my understadning is that the intermediate state is something
> > > > that
> > > > we need to avoid (hard to be migrated anyhow).
> > > The difficulty is on the device side though, so why not say: if it wants to
> > > flush IO that's up to it.
> > Do you mean to introduce a device specific way to flush IO? If yes, it
> > actually introduces a new intermediate state implicitly:
> >
> > 1) device is stopped
> > 2) device is stopped and IO is flushed
> >
> > This looks more complicated than a single new state:
> >
> > 1) device is stopped and IO is flushed
> >
> > Thanks
> I thought some more about it, if we call a buffer which was
> made available by driver but not yet used by device,


For "used by device" you meant posting entries to used ring?


> a cancelled buffer, then maybe we can specify
> the behaviour by explaining that it should be possible to
> retry any cancelled buffer, that is make it available again,
> and achieve the same effect as if it was used.
> For example, any reads without side effects can be just
> ignored even if they might have modified guest memory.


Right, but how about the buffer that can't be retried? Or you meant we 
leave that to a specific device to handle?

THanks


>