Re: [PULL 18/28] file-posix: try BLKSECTGET on block devices too, do not round to power of 2

[Halil Pasic]

On Fri, 25 Jun 2021 16:18:12 +0200
Paolo Bonzini <pbonzini@redhat.com> wrote:

> bs->sg is only true for character devices, but block devices can also
> be used with scsi-block and scsi-generic.  Unfortunately BLKSECTGET
> returns bytes in an int for /dev/sgN devices, and sectors in a short
> for block devices, so account for that in the code.
> 
> The maximum transfer also need not be a power of 2 (for example I have
> seen disks with 1280 KiB maximum transfer) so there's no need to pass
> the result through pow2floor.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

We have found that this patch leads to in guest I/O errors when DASD
is used as a source device. I.e. libvirt domain xml wise something like:

    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='none' io='native' iothread='1'/>
      <source dev='/dev/disk/by-id/ccw-XXXXXXX'/>
      <backingStore/>
      <target dev='vdb' bus='virtio'/>
      <address type='ccw' cssid='0xfe' ssid='0x0' devno='0x0008'/>
    </disk>

I don't think it is the fault of this patch: it LGTM. But it correlates
100%, furthermore the problem seems to be related to the value of
bl.max_iov which now comes from sysfs. 

We are still investigating what is actually wrong. Just wanted to give
everybody a heads-up that this does seem to cause a nasty regression on
s390x, even if the code itself is perfect.

Regards,
Halil