[PATCH v1 0/1] virtio-block: switch to blk_get_max_hw_transfer

[Or Ozeri]

Since moving to qemu 6.1.0 we've been seeing disk failures inside a qemu guest 
VM.
This happened when using if=virtio on a host /dev/nbdX device.
Binary searching on qemu commit history, I found these errors start on this 
commit:

Commit 18473467
file-posix: try BLKSECTGET on block devices too, do not round to power of 2

The above commit switched posix block device limits (including host /dev/nbdX 
devices)
to query limits from /sys/dev/block/..., instead of using predefined limits.

The scsi-generic driver was changed to use the queried limits,
whereas the virtio-blk driver was only the queried max_iov,
but still using the predefined max_transfer, which is unlimited in qemu.

For NBD devices, max_iov is unlimited by the kernel nbd driver.
As as consequence, the virtio-blk merged requests over the limit of our host 
/dev/nbdX device,
which apparently caused the guest disk errors.

The solution that worked for me was to change the virtio-blk driver to use the 
max_transfer
limit queried from the posix host device (given by blk_get_max_hw_transfer).

Or Ozeri (1):
  virtio-block: switch to blk_get_max_hw_transfer

 hw/block/virtio-blk.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

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2.25.1