linux-kernel - Re: [RFC] virtio_scsi: to poll and kick the virtqueue in timeout handler

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Message-ID: <YKupFeOtc6Pr5KS2@stefanha-x1.localdomain>
Date:   Mon, 24 May 2021 14:24:37 +0100
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Hannes Reinecke <hare@...e.de>
Cc:     Dongli Zhang <dongli.zhang@...cle.com>,
        virtualization@...ts.linux-foundation.org,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org, mst@...hat.com, jasowang@...hat.com,
        pbonzini@...hat.com, jejb@...ux.ibm.com,
        martin.petersen@...cle.com, joe.jin@...cle.com,
        junxiao.bi@...cle.com, srinivas.eeda@...cle.com
Subject: Re: [RFC] virtio_scsi: to poll and kick the virtqueue in timeout
 handler

On Sun, May 23, 2021 at 09:39:51AM +0200, Hannes Reinecke wrote:
> On 5/23/21 8:38 AM, Dongli Zhang wrote:
> > This RFC is to trigger the discussion about to poll and kick the
> > virtqueue on purpose in virtio-scsi timeout handler.
> > 
> > The virtio-scsi relies on the virtio vring shared between VM and host.
> > The VM side produces requests to vring and kicks the virtqueue, while the
> > host side produces responses to vring and interrupts the VM side.
> > 
> > By default the virtio-scsi handler depends on the host timeout handler
> > by BLK_EH_RESET_TIMER to give host a chance to perform EH.
> > 
> > However, this is not helpful for the case that the responses are available
> > on vring but the notification from host to VM is lost.
> > 
> How can this happen?
> If responses are lost the communication between VM and host is broken, and
> we should rather reset the virtio rings themselves.

I agree. In principle it's fine to poll the virtqueue at any time, but I
don't understand the failure scenario here. It's not clear to me why the
device-to-driver vq notification could be lost.

Stefan

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