Re: [PATCH v4 3/7] block: add max_hw_transfer to BlockLimits

[Eric Blake <eblake@redhat.com>]

On Tue, Jun 08, 2021 at 03:16:30PM +0200, 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 +

[you can use git's orderfile option to put .h changes first]

>  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)

Since we have declared (elsewhere) that the maximum block device is
signed, would this be better as int64_t?  (Our reasoning is that off_t
is also signed, and we are unlikely to need to handle anything bigger
than what off_t can handle; plus it leaves room for returning errors,
although this particular function is not giving errors; see also
Vladimir's work on making the block layer 64-bit clean).  I'm not
opposed to unsigned here to represent lack of errors, but consistency
with the rest of the block layer may argue for signed.

> +++ b/include/block/block_int.h
> @@ -695,6 +695,13 @@ typedef struct BlockLimits {
>       * clamped down. */
>      uint32_t max_transfer;
>  
> +    /* Maximal hardware transfer length in bytes.  Applies whenever
> +     * transfers to the device bypass the kernel I/O scheduler, for
> +     * example with SG_IO.  If larger than max_transfer or if zero,
> +     * blk_get_max_hw_transfer will fall back to max_transfer.
> +     */
> +    uint64_t max_hw_transfer;
> +

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org