Re: [PATCH] block/vhost-user-blk: Fix hang on boot for some odd guests

[Andrey Ryabinin]

On 4/18/23 07:13, Raphael Norwitz wrote:
> Hey Andrey - apologies for the late reply here.
> 
> It sounds like you are dealing with a buggy guest, rather than a QEMU issue.

No arguing here, the guest is buggy.
However, the issue with QEMU is that virtio-blk tolerate such buggy guest
while vhost-user-blk is not.
We've been using virtio-blk in our cloud for a while and recently started 
switching to vhost-user-blk
which led us to discover this problem.

>> On Apr 10, 2023, at 11:39 AM, Andrey Ryabinin <arbn@yandex-team.com> wrote:
>>
>>
>>
>> On 4/10/23 10:35, Andrey Ryabinin wrote:
>>> Some guests hang on boot when using the vhost-user-blk-pci device,
>>> but boot normally when using the virtio-blk device. The problem occurs
>>> because the guest advertises VIRTIO_F_VERSION_1 but kicks the virtqueue
>>> before setting VIRTIO_CONFIG_S_DRIVER_OK, causing vdev->start_on_kick to
> 
> Virtio 1.1 Section 3.1.1, says during setup “[t]he driver MUST NOT notify the 
> device before setting DRIVER_OK.”
> 
> Therefore what you are describing is buggy guest behavior. Sounds like the 
> driver should be made to either
> - not advertise VIRTIO_F_VERSION_1
> - not kick before setting VIRTIO_CONFIG_S_DRIVER_OK
> 
> If anything, the virtio-blk virtio_blk_handle_output() function should 
> probably check start_on_kick?
> 

Ideally this should have been done from the start. But if we do it now we'll 
just break these guests.