On Wed, Jul 12, 2023 at 11:05:11AM -0500, Mike Christie wrote: > On 7/12/23 9:26 AM, Stefan Hajnoczi wrote: > > 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. > > I don't think you can just change the driver's behavior to fail now, > because apps send mis-aligned IO and its working as long as they have less > than 256 bio vecs. > > We see mis-aligned IOs during boot and also from random non window's apps. > If we just start to fail then it would be a regression when the app no > longer works or the OS fails to start up. I was wrong: The virtio-scsi specification contains no alignment requirements for I/O buffers. It is fine for the driver to submit iovecs with any memory alignment. The QEMU code allocates a bounce buffer if the iovecs submitted by the driver do not match the minimum alignment requirements on the host (e.g. O_DIRECT requirements). It makes sense that vhost_scsi needs to use a bounce buffer in cases where the underlying storage has stricter memory alignment requirements. Acked-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
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