Re: [Qemu-devel] [PATCH] scsi-disk: Don't enlarge min_io_size to max_io_size

[Fam Zheng <famz@redhat.com>]

On Thu, 03/22 09:19, Daniel Henrique Barboza wrote:
> Hi,
> 
> On 03/22/2018 04:38 AM, Fam Zheng wrote:
> > Some backends report big max_io_sectors. Making min_io_size the same
> > value in this case will make it impossible for guest to align memory,
> > therefore the disk may not be usable at all.
> > 
> > Change the default behavior (when min_io_size and opt_io_size are not
> > specified in the command line), do not assume max_io_sectors is a good
> > value for opt_io_size and min_io_size, use 512 instead.
> > 
> > Reported-by: David Gibson <dgibson@redhat.com>
> > Signed-off-by: Fam Zheng <famz@redhat.com>
> > ---
> >   hw/scsi/scsi-disk.c | 6 ++----
> >   1 file changed, 2 insertions(+), 4 deletions(-)
> > 
> > diff --git a/hw/scsi/scsi-disk.c b/hw/scsi/scsi-disk.c
> > index 5b7a48f5a5..76e3c9eaa4 100644
> > --- a/hw/scsi/scsi-disk.c
> > +++ b/hw/scsi/scsi-disk.c
> > @@ -714,10 +714,8 @@ static int scsi_disk_emulate_inquiry(SCSIRequest *req, uint8_t *outbuf)
> > 
> >                   /* min_io_size and opt_io_size can't be greater than
> >                    * max_io_sectors */
> > -                min_io_size =
> > -                    MIN_NON_ZERO(min_io_size, max_io_sectors);
> > -                opt_io_size =
> > -                    MIN_NON_ZERO(opt_io_size, max_io_sectors);
> > +                min_io_size = MIN(min_io_size ? : 512, max_io_sectors);
> > +                opt_io_size = MIN(opt_io_size ? : 512, max_io_sectors);
> >               }
> 
> This code you're changing was added in d082d16a5c ("consider
> bl->max_transfer ..").
> I've borrowed this logic from scsi-generic.c, scsi_read_complete:
> 
>     if (s->type == TYPE_DISK &&
>         r->req.cmd.buf[0] == INQUIRY &&
>         r->req.cmd.buf[2] == 0xb0) {
>         uint32_t max_transfer =
>             blk_get_max_transfer(s->conf.blk) / s->blocksize;
> 
>         assert(max_transfer);
>         stl_be_p(&r->buf[8], max_transfer);
>         /* Also take care of the opt xfer len. */
>         stl_be_p(&r->buf[12],
>                  MIN_NON_ZERO(max_transfer, ldl_be_p(&r->buf[12])));
>     }
> 
> 
> Unless I've misunderstood the bug, you will want to change this code too.
> Otherwise
> you'll fix it with emulated disks but it might appear when using SCSI
> passthrough.

I am assuming (because I don't have a reproducer myself) what matters is
min_io_size here.

David, could you help test if you see the same problem with "-device
scsi-block"? If we I'll patch scsi-generic.c in v2 too.

Fam