Re: [PATCH v2 0/2] vhost-scsi: Fix IO hangs when using windows

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Jul 11, 2023 at 04:01:22PM -0500, Mike Christie wrote:
> On 7/11/23 1:34 PM, Stefan Hajnoczi wrote:
> > On Sun, Jul 09, 2023 at 03:28:57PM -0500, Mike Christie wrote:
> >> The following patches were made over Linus's tree and fix an issue
> >> where windows guests will send iovecs with offset/lengths that result
> >> in IOs that are not aligned to 512. The LIO layer will then send them
> >> to Linux's FS/block layer but it requires 512 byte alignment, so
> >> depending on the FS/block driver being used we will get IO errors or
> >> hung IO.
> >>
> >> The following patches have vhost-scsi detect when windows sends these
> >> IOs and copy them to a bounce buffer. It then does some cleanup in
> >> the related code.
> > 
> > Hang on, virtio-scsi is a SCSI HBA and READs/WRITEs submitted must
> > follow the usual constraints on SCSI block limits. Would Windows send
> > mis-aligned I/O to a non-virtio-scsi SCSI HBA?
> 
> It's like linux where you can config settings like that.
> 
> > > Are you sure this is not a bug in the Windows guest driver where block
> > limits are being misconfigured?
> 
> From what our windows dev told us the guest drivers like here:
> 
> https://github.com/virtio-win
> 
> don't set the windows AlignmentMask to 512. They tried that and it
> resulted in windows crash dump crashing because it doesn't like the
> hard alignment requirement.
> 
> We thought other apps would have trouble as well, so we tried to add
> bounce buffer support to the windows driver, but I think people thought
> it was going to be uglier than this patch and in the normal alignment
> case might also affect performance. There was some windows driver/layering
> and buffer/cmd details that I don't fully understand and took their word
> for because I don't know a lot about windows.
> 
> In the end we still have to add checks to vhost-scsi to protect against
> bad drivers, so we thought we might as well just add bounce buffer support
> to vhost-scsi.

CCing virtio-win developers so they can confirm how the vioscsi driver
is supposed to handle request alignment.

My expectation is that the virtio-scsi device will fail mis-aligned I/O
requests.

Stefan

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