Re: [PATCH v2 05/11] hw/block: reject a zoned device whose write pointers are unaddressable

[Damien Le Moal <dlemoal@kernel.org>]

On 9/3/26 04:44, Niklas Cassel wrote:
> The write pointers of a zoned device outlive any particular use of it,
> whether the device keeps them itself or a backend records them, while the
> logical block size is a property of the frontend and is chosen afresh
> every time the device is attached. Nothing ties the two together. A zone
> written while the device was configured with logical_block_size=512
> leaves a write pointer that is a multiple of 512, and attaching the same
> device with logical_block_size=4096 makes that pointer unaddressable.
> 
> Such a pointer is not merely misaligned. The guest addresses the device
> in logical blocks, and a zone report expresses the write pointer in 512
> byte sectors, so the guest is told about a position that does not fall on
> a logical block boundary. It can neither read nor write there, and the
> zone can only be recovered by resetting it. The reverse direction is
> harmless: a pointer laid down with a larger logical block size is still a
> multiple of a smaller one.
> 
> On a zoned null_blk device with a logical block size of 512, a 512 byte
> append to a sequential zone leaves the write pointer half a logical block
> into it:
> 
>   $ qemu-io --image-opts -n driver=host_device,filename=/dev/nullb0 \
>         -c "zap -p 0x20000000 0x200" -c "zrp 0x20000000 1"
>   start: 0x100000, len 0x80000, cap 0x80000, wptr 0x100001, zcond:2
> 
> Attaching that disk with logical_block_size=4096 handed the guest a zone
> it could not write to.
> 
> Check at realize time that the zone size and every write pointer of a
> sequential zone are multiples of the write granularity that the device is
> about to report, 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>

Looks good to me, but I think a better name for the helper should be
blkconf_check_zoned_geometry(), to make it clear what that function does.
With that fixed, feel free to add:

Reviewed-by: Damien Le Moal <dlemoal@kernel.org>

-- 
Damien Le Moal
Western Digital Research