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

[Eric Blake <eblake@redhat.com>]

On Thu, Jun 03, 2021 at 03:37:18PM +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.
> 

> +/* 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;

This is an unaligned value; should we instead round it down to the
request_alignment granularity?

> +
> +    if (bs) {
> +        max = MIN_NON_ZERO(bs->bl.max_hw_transfer, bs->bl.max_transfer);
> +    }
> +    return max;
> +}
> +
>  /* Returns the maximum transfer length, in bytes; guaranteed nonzero */
>  uint32_t blk_get_max_transfer(BlockBackend *blk)
>  {

> +++ 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

Leading /* on its own line, per our style.

> +     * 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.
> +     */

Should we mandate any additional requirements on this value such as
multiple of request_alignment or even power-of-2?

> +    uint64_t max_hw_transfer;
> +
>      /* memory alignment, in bytes so that no bounce buffer is needed */
>      size_t min_mem_alignment;
>  

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