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

[Stefan Hajnoczi <stefanha@redhat.com>]

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?

> + * 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).

> +	if (virtscsi_init(vdev, vscsi))
> +		return FAILED;
> +
> +	virtio_device_ready(vdev);

Missing virtscsi_kick_event_all() like in virtscsi_restore()?

> +	return SUCCESS;
> +}
> +
>  static int virtscsi_probe(struct virtio_device *vdev)
>  {
>  	struct Scsi_Host *shost;
> -- 
> 2.43.0
>