Re: [PATCH v3 3/7] block: add max_hw_transfer to BlockLimits

[Max Reitz]

On 03.06.21 15:37, Paolo Bonzini wrote:
> For block host devices, I/O can happen through either the kernel file
> descriptor I/O system calls (preadv/pwritev, io_submit, io_uring)
> or the SCSI passthrough ioctl SG_IO.
>
> In the latter case, the size of each transfer can be limited by the
> HBA, while for file descriptor I/O the kernel is able to split and
> merge I/O in smaller pieces as needed.  Applying the HBA limits to
> file descriptor I/O results in more system calls and suboptimal
> performance, so this patch splits the max_transfer limit in two:
> max_transfer remains valid and is used in general, while max_hw_transfer
> is limited to the maximum hardware size.  max_hw_transfer can then be
> included by the scsi-generic driver in the block limits page, to ensure
> that the stricter hardware limit is used.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>   block/block-backend.c          | 12 ++++++++++++
>   block/file-posix.c             |  2 +-
>   block/io.c                     |  1 +
>   hw/scsi/scsi-generic.c         |  2 +-
>   include/block/block_int.h      |  7 +++++++
>   include/sysemu/block-backend.h |  1 +
>   6 files changed, 23 insertions(+), 2 deletions(-)
>
> diff --git a/block/block-backend.c b/block/block-backend.c
> index 15f1ea4288..2ea1412a54 100644
> --- a/block/block-backend.c
> +++ b/block/block-backend.c
> @@ -1953,6 +1953,18 @@ uint32_t blk_get_request_alignment(BlockBackend *blk)
>       return bs ? bs->bl.request_alignment : BDRV_SECTOR_SIZE;
>   }
>   
> +/* Returns the maximum hardware transfer length, in bytes; guaranteed nonzero */
> +uint64_t blk_get_max_hw_transfer(BlockBackend *blk)
> +{
> +    BlockDriverState *bs = blk_bs(blk);
> +    uint64_t max = INT_MAX;
> +
> +    if (bs) {
> +        max = MIN_NON_ZERO(bs->bl.max_hw_transfer, bs->bl.max_transfer);
> +    }
> +    return max;

Both `max_hw_transfer` and `max_transfer` can be 0, so this could return 
0, contrary to what the comment above promises.

Should `max` be initialized to 0 with a `MIN_NON_ZERO(max, INT_MAX)` 
here (like `blk_get_max_transfer()` does it)?

(As for the rest, I think aligning to the request alignment makes sense, 
but then again we don’t do that for max_transfer either, so... this at 
least wouldn’t be a new bug.

Regarding the comment, checkpatch complains about it, so it should be 
fixed so that /* is on its own line.

Speaking of checkpatch, now that I ran it, it also complains about the 
new line in bdrv_merge_limits() exceeding 80 characters, so that should 
be fixed, too.)

Max