Re: [PATCH] virtio: skip guest index check on device load

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Oct 28, 2020 at 11:00:38AM +0000, Stefan Hajnoczi wrote:
> On Tue, Oct 27, 2020 at 09:04:46AM -0400, Michael S. Tsirkin wrote:
> > It's not a waste of time, it's just a lot of work
> > within guests.
> 
> Luckily it does no harm to set the NEEDS_RESET bit even if the guest
> doesn't handle it.
> 
> If the guest driver is unaware it may continue to submit requests to the
> device for a while. The device emulation code stops accepting new
> requests though. This means the device will become unresponsive until
> reset, which is not ideal but okay in the case where the device was put
> into an invalid state.

There is no actual rule that device must stop processing requests.
Driver can only assume that is the case after it started the
actual reset.


> I agree that supporting NEEDS_RESET transparently inside guests is
> difficult. The driver needs to reset and resume the device without
> reporting errors to applications. In some cases drivers may not have
> enough state in order to do that. It's also tricky to test all code
> paths. I guess this is why no one has done it: drivers shouldn't enter
> the NEEDS_RESET state anyway and handling it is complex.
> 
> Stefan