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block: add max_hw_transfer to BlockLimits

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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>
This commit is contained in:
Paolo Bonzini 2021-06-03 10:34:23 +02:00
parent b99f7fa08a
commit 24b36e9813
6 changed files with 25 additions and 2 deletions
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13
block/block-backend.c
View file

@ -1953,6 +1953,19 @@ 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(max, bs->bl.max_hw_transfer);
max = MIN_NON_ZERO(max, bs->bl.max_transfer);
}
return ROUND_DOWN(max, blk_get_request_alignment(blk));
}
/* Returns the maximum transfer length, in bytes; guaranteed nonzero */
uint32_t blk_get_max_transfer(BlockBackend *blk)
{