Regression in xen-blkfront regarding sector sizes

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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Hi,

Some Qubes users report a regression in xen-blkfront regarding block
size reporting. It works fine on 6.8.8, but appears broken on 6.9.2.

The specific problem is that blkfront reports block size of 512, even for
backend devices of 4096. This, for example, fails 512-bytes reads with
O_DIRECT, and appears to break mounting a filesystem on such a device
(at least xfs one).

For example it looks like this:

    [user@dom0 ~]$ head /sys/block/loop12/queue/*_block_size
    ==> /sys/block/loop12/queue/logical_block_size <==
    4096

    ==> /sys/block/loop12/queue/physical_block_size <==
    4096

    [user@dom0 bin]$ qvm-run -p the-vm 'head /sys/block/xvdi/queue/*_block_size'
    ==> /sys/block/xvdi/queue/logical_block_size <==
    512

    ==> /sys/block/xvdi/queue/physical_block_size <==
    512

and then:

    $ sudo dd if=/dev/xvdi of=/dev/null count=1 status=progress iflag=direct
    /usr/bin/dd: error reading '/dev/xvdi': Input/output error
    0+0 records in
    0+0 records out
    0 bytes copied, 0.000170858 s, 0.0 kB/s

and mounting fails like this:

    [   68.055045] SGI XFS with ACLs, security attributes, realtime, scrub, quota, no debug enabled
    [   68.057308] I/O error, dev xvdi, sector 0 op 0x0:(READ) flags 0x1000 phys_seg 1 prio class 0
    [   68.057333] XFS (xvdi): SB validate failed with error -5.

More details at https://github.com/QubesOS/qubes-issues/issues/9293

Rusty suspects it's related to
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/block/xen-blkfront.c?id=ba3f67c1163812b5d7ec33705c31edaa30ce6c51,
so I'm cc-ing people mentioned there too.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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