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Date:   Mon, 24 May 2021 14:24:37 +0100
From:   Stefan Hajnoczi <stefanha@redhat.com>
To:     Hannes Reinecke <hare@suse.de>
Cc:     Dongli Zhang <dongli.zhang@oracle.com>,
        virtualization@lists.linux-foundation.org,
        linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-block@vger.kernel.org, mst@redhat.com, jasowang@redhat.com,
        pbonzini@redhat.com, jejb@linux.ibm.com,
        martin.petersen@oracle.com, joe.jin@oracle.com,
        junxiao.bi@oracle.com, srinivas.eeda@oracle.com
Subject: Re: [RFC] virtio_scsi: to poll and kick the virtqueue in timeout
 handler
Message-ID: <YKupFeOtc6Pr5KS2@stefanha-x1.localdomain>
References: <20210523063843.1177-1-dongli.zhang@oracle.com>
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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.
> >=20
> > 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 t=
he
> > host side produces responses to vring and interrupts the VM side.
> >=20
> > 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.
> >=20
> > However, this is not helpful for the case that the responses are availa=
ble
> > on vring but the notification from host to VM is lost.
> >=20
> 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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