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

[David Gibson <dgibson@redhat.com>]

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On Mon, 26 Mar 2018 15:26:39 +0800
Fam Zheng <famz@redhat.com> wrote:

> 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)

Sorry, I should have given you specific reproduce instructions.  You
don't need a POWER host - I've verified that the bug trips under TCG.

  1. Grab a RHEL ppc64le install image (other installers could well
     also hit it, but I haven't tried them)
  2. Build current qemu master, including the ppc64-softmmu target
  3. Create a fresh new guest disk image
        qemu-img create -f qcow2 disk.qcow2 20G
  4. Attempt to install the new guest:
        $QEMU -nodefaults -nographic -machine pseries \
              -cpu POWER8 -smp 1 -m 1G \
              -chardev stdio,id=conmon,mux=on,signal=off \
              -device spapr-vty,chardev=conmon \
              -mon conmon \
              -device virtio-scsi-pci,id=scsi \
              -drive file=disk.qcow2,if=none,format=qcow2,id=hd \
              -device scsi-disk,drive=hd,bus=scsi.0 \
              -drive file=RHEL-7.4-20170711.0-Server-ppc64le-dvd1.iso,format=raw,media=cdrom,if=none,id=cd \
              -device scsi-cd,drive=cd,bus=scsi.0

That's using the RHEL7.4 GA image, a recent 7.5 snapshot also works as may others.

> 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.

I'm not sure exactly what you want me to check here?  You mean putting
the guest disk on a scsi-block instead of scsi-disk?  That's a bit more
fiddly, since I have to find a block device to back it instead of an
image.

-- 
David Gibson <dgibson@redhat.com>
Principal Software Engineer, Virtualization, Red Hat

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