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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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2
block/file-posix.c
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@ -1232,7 +1232,7 @@ static void raw_refresh_limits(BlockDriverState *bs, Error **errp)
int ret = sg_get_max_transfer_length(s->fd);
if (ret > 0 && ret <= BDRV_REQUEST_MAX_BYTES) {
bs->bl.max_transfer = pow2floor(ret);
bs->bl.max_hw_transfer = pow2floor(ret);
}
ret = sg_get_max_segments(s->fd);