Re: [Qemu-devel] [RFC] scsi: restart dma after vm change state handlers

[Kevin Wolf <kwolf@redhat.com>]

Am 21.05.2019 um 13:04 hat Paolo Bonzini geschrieben:
> On 21/05/19 12:36, Stefan Hajnoczi wrote:
> > This is RFC because I am waiting for a test result on the system where
> > the bug was originally discovered.  I'm also open to nicer solutions!
> 
> I don't think it's too ugly; IDE is also using a bottom half for this.

I think the IDE case is different, see commit 213189ab65d. The case
we're protecting against there is stopping the VM from inside a VM state
handler, which can confuse other VM state callbacks that come later. The
actual order of the IDE callback vs. the other callback doesn't matter,
it's just important that all start callbacks are completed before stop
callbacks are called.

In our current case, the problem is not that we're confusing other
handlers, but that we rely on another handler to have completed resuming
something. If that other handler changes e.g. to use a BH itself, we get
an undefined order again.

The clean solution would probably be not to use a VM state handler in
scsi-bus, but a callback from the HBA that tells the bus that the HBA is
ready to receive requests again.

If we go with the not so clean solution, maybe at least a comment in
virtio-scsi would be in order.

Kevin