Re: [PATCH v6 07/15] hw/block: reject a zoned device that the block sizes cannot address

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Sep 21, 2026 at 04:28:09PM +0200, Niklas Cassel wrote:
> The write granularity that a frontend reports is derived from the logical
> and the physical block size, which are properties of the device model,
> while the granularity that the medium requires and the write pointers
> that it holds belong to the backend. Nothing ties them together, and
> where they disagree a guest is told about a device it cannot use.
> 
> A backend can require a coarser granularity than the block sizes express.
> A 512e host managed disk has a logical block size of 512 and requires
> writes to a sequential zone to be a multiple of 4096, so attaching it
> with a finer physical block size reports a granularity that the disk will
> not accept, and the guest is given a plain I/O error for a request that
> it was told was valid.
> 
> The write pointers can disagree in the same way. They outlive any
> particular use of the device, while the block sizes are chosen afresh
> every time it is attached, so a zone written with logical_block_size=512
> leaves a write pointer that logical_block_size=4096 cannot address. A
> zone report expresses it in 512 byte sectors, so the guest is told about
> a position that does not fall on a logical block boundary, which it can
> neither read nor write, and the zone can then only be recovered by
> resetting it. The reverse is harmless: a pointer laid down with a larger
> logical block size is still a multiple of a smaller one.
> 
> Check both at realize time, along with the zone size and the zone
> capacity, and refuse to start otherwise. The write pointers are already
> held in memory by the driver, so this costs no I/O.
> 
> The check uses blkconf_zone_write_granularity(), the same value that a
> frontend reports to its guest and validates requests against, so the
> three cannot disagree.
> 
> Signed-off-by: Niklas Cassel <cassel@kernel.org>
> ---
>  hw/block/block.c         | 63 ++++++++++++++++++++++++++++++++++++++++
>  hw/block/virtio-blk.c    |  4 +++
>  include/hw/block/block.h |  1 +
>  3 files changed, 68 insertions(+)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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