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

[Hannes Reinecke <hare@suse.de>]

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?

>> + * reset also times out. A virtio reset guarantees the device won't
>> + * touch guest memory again, so stuck commands can be freed safely.
>> + * Same teardown/rebuild already used for suspend/resume.
>> + */
>> +static int virtscsi_host_reset(struct scsi_cmnd *sc)
>> +{
>> +	struct virtio_scsi *vscsi = shost_priv(sc->device->host);
>> +	struct virtio_device *vdev = vscsi->vdev;
>> +
>> +	scmd_printk(KERN_INFO, sc, "host reset\n");
>> +
>> +	virtscsi_remove_vqs(vdev);
> 
> This is synchronous and may take a long time. I noticed that some SCSI
> drivers have a timeout that returns FAILED when the low-level host reset
> operation takes too long. This can be done here too but would require
> adding a timeout to virtio_reset_device() (i.e. see the virtio-pci
> implementation that loops until the device register indicates reset is
> complete).
> 
Never, ever, return FAILED from a host reset.
That will induce a non-recoverable error, and you have to reset the
OS to get the virtio devices back.
Not something you'd want to do I guess.

Yes, I know that host reset can take a long time. But normally
host reset would be the last resort, so really when you have to
call it your system has exhausted all other options.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
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