Re: [PATCH v2] scsi: virtio_scsi: implement eh_host_reset_handler

[Stefan Hajnoczi <stefanha@redhat.com>]

On Fri, Nov 06, 2026 at 02:12:19PM -0600, Mike Christie wrote:
> On 11/3/26 7:51 AM, Stefan Hajnoczi wrote:
> > On Tue, Nov 03, 2026 at 08:27:52AM +0100, Hannes Reinecke wrote:
> >> On 11/2/26 9:06 PM, Stefan Hajnoczi wrote:
> >>> On Mon, Sep 14, 2026 at 08:31:44PM +0700, Nguyen Ngoc Thang wrote:
> >>>> Once virtscsi_eh_timed_out() lets SCSI EH run on an unresponsive host,
> >>>> abort and device reset both fail the same way (same dead ctrl vq), and
> >>>> since virtio_scsi implements neither target nor bus reset, EH falls
> >>>> through to scsi_eh_offline_sdevs(), which frees the stuck commands' DMA
> >>>> buffers without any guarantee the device has actually stopped touching
> >>>> them. If bus mastering (or whatever broke the transport) comes back
> >>>> later, the device can still write into memory the kernel has already
> >>>> reused.
> >>>>
> >>>> Add a real eh_host_reset_handler instead. virtio_reset_device() is the
> >>>> guarantee scsi_eh_offline_sdevs() was missing: once it returns, the
> >>>> device will not touch guest memory again, so it is safe to hand
> >>>> outstanding commands' buffers back. Reuse virtscsi_remove_vqs() +
> >>>> virtscsi_init() -- the exact sequence already used across
> >>>> suspend/resume -- to tear the virtqueues down and rebuild them.
> >>>>
> >>>> Verified with a QEMU virtio-scsi repro (PCI_COMMAND_MASTER cleared
> >>>> mid-write): EH now runs abort -> device reset -> host reset -> abort ->
> >>>> device reset -> host reset -> offline, then dd's stuck fsync returns
> >>>> EIO and D-state drains to 0, in ~244s total. No task remains
> >>>> uninterruptibly blocked.
> >>> By the way, there is a known limitation in the QEMU virtio-scsi
> >>> implementation where some reset-related operations are implemented
> >>> synchronously (can block vCPUs and QEMU activity while waiting for I/O
> >>> to drain in some situations) instead of asynchronously. Recently there
> >>> has been work in QEMU to make VIRTIO device reset asynchronous, for
> >>> example, but it needs to be implemented on a per-device basis and
> >>> virtio-scsi hasn't taken advantage of it yet.
> >>>
> >>> Please let me know if you'd like to discuss or contribute. It's
> >>> something that is on my radar but not until 2027.
> >>>
> >>>> Suggested-by: Hannes Reinecke <hare@suse.de>
> >>>> Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>
> >>>> ---
> >>>> v1 -> v2: implemented eh_host_reset_handler as suggested by Hannes
> >>>> Reinecke in review of the original RFC, instead of relying solely on
> >>>> the EH loop falling through to offline.
> >>>> ---
> >>>>   drivers/scsi/virtio_scsi.c | 24 ++++++++++++++++++++++++
> >>>>   1 file changed, 24 insertions(+)
> >>>>
> >>>> diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
> >>>> index b4f20c487718..9698f4f91cc6 100644
> >>>> --- a/drivers/scsi/virtio_scsi.c
> >>>> +++ b/drivers/scsi/virtio_scsi.c
> >>>> @@ -833,6 +833,8 @@ static enum scsi_timeout_action virtscsi_eh_timed_out(struct scsi_cmnd *scmnd)
> >>>>   	return SCSI_EH_NOT_HANDLED;
> >>>>   }
> >>>> +static int virtscsi_host_reset(struct scsi_cmnd *sc);
> >>>> +
> >>>>   static const struct scsi_host_template virtscsi_host_template = {
> >>>>   	.module = THIS_MODULE,
> >>>>   	.name = "Virtio SCSI HBA",
> >>>> @@ -846,6 +848,7 @@ static const struct scsi_host_template virtscsi_host_template = {
> >>>>   	.eh_abort_handler = virtscsi_abort,
> >>>>   	.eh_device_reset_handler = virtscsi_device_reset,
> >>>>   	.eh_timed_out = virtscsi_eh_timed_out,
> >>>> +	.eh_host_reset_handler = virtscsi_host_reset,
> >>>>   	.sdev_init = virtscsi_device_alloc,
> >>>>   	.dma_boundary = UINT_MAX,
> >>>> @@ -947,6 +950,27 @@ static int virtscsi_init(struct virtio_device *vdev,
> >>>>   	return err;
> >>>>   }
> >>>> +/*
> >>>> + * No bus/target reset in virtio-scsi, so EH lands here after device
> >>> The choice to use the VIRTIO device reset in virtscsi_host_reset() looks
> >>> good to me, but I would like clarification on this comment. virtio-scsi
> >>> does have I_T_NEXUS_RESET and I believe this is the modern equivalent to
> >>> the pre-SCSI-6 TARGET RESET. Is it really the case that virtio-scsi
> >>> doesn't have target reset? And do you want to go ahead and implement
> >>> .eh_target_reset_handler() using I_T_NEXUS_RESET?
> >>>
> >> Would probably not a bad idea to use I_T_NEXUS_RESET here.
> >> Not sure what it does internally, though; _is_ there a nexus
> >> to reset?
> > Whether there is any benefit to I_T_NEXUS_RESET may depend on the device
> > implementation. In QEMU:
> > - LOGICAL_UNIT_RESET resets just 1 LUN
> > - I_T_NEXUS_RESET resets all LUNs
> > - VIRTIO device reset also resets the HBA itself
> > 
> > In practice QEMU's I_T_NEXUS_RESET is unlikely to fix things if
> > LOGICAL_UNIT_RESET wasn't sufficient. Maybe it will help on other device
> > implementations though.
> > 
> > I think the Linux driver mapping should be:
> > - .eh_device_reset_handler() -> LOGICAL_UNIT_RESET
> > - .eh_target_reset_handler() -> I_T_NEXUS_RESET
> > - .eh_host_reset_handler() -> VIRTIO device reset
> 
> How are vhost drivers notified of the device reset? For qemu virtio-scsi
> this would call virtio_scsi_reset right?
> 
> For vhost-scsi though, do we need to add a similar reset function?

VIRTIO device reset is handled by vhost_scsi's VHOST_SCSI_CLEAR_ENDPOINT
and VHOST_SCSI_SET_ENDPOINT ioctls called from QEMU's
hw/scsi/vhost-scsi.c:vhost_scsi_set_status(). The Linux vhost_scsi code
flushes requests and resets the device, so I don't think code changes
are needed.

Stefan