Re: [PATCH] vhost-scsi: flush backend after device ioctls

[Jia Jia <physicalmtea@gmail.com>]

The changelog is too long. I wrote it incrementally while investigating
the stale response-HVA case. English is not my first language, so some parts
came out wrong. In particular, saying that the ioctl waits for pre-update
commands may have suggested a full stop-new -> flush -> swap -> start-new
sequence. That was not what I meant; sorry about the confusion. If the patch
is otherwise acceptable, I will shorten the changelog in v2.

I wrote the patch myself. I used AI assistance only for notes and wording
help; it did not author or submit the code.

For this specific stale-response-HVA issue, I believe the patch is correct.
The patch is a return barrier: vhost_dev_ioctl() publishes the new memory
table, and vhost_scsi_flush() switches each vhost virtqueue to a new
inflight generation, flushes the vhost work, and waits for the old
generation's references. The completion path copies the response through
cmd->tvc_resp_iovs before releasing the old-generation reference. There is
no separate stop-new phase or full quiesce.

A command can arrive after vhost_dev_ioctl() returns and before
vhost_scsi_flush() switches that virtqueue's generation. It is assigned to the
old generation and is included in the flush, but vhost_set_memory() has
already updated that virtqueue's memory table, so its response iov uses the
new table. It therefore does not introduce another stale-HVA case. This is
why stopping new commands is not needed here.

An old-generation command may complete while this ioctl is still running in
the kernel, including while it is blocked in vhost_scsi_flush(); that is
expected. The same userspace thread cannot perform the remap and follow-up
TUR before this ioctl returns, because the thread is still blocked inside the
ioctl. The owner must keep the old mappings valid until the ioctl returns.
Remapping or dropping them from another userspace thread before then is
outside the lifetime assumption of this transition barrier.