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block: introduce max_hw_iov for use in scsi-generic

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Linux limits the size of iovecs to 1024 (UIO_MAXIOV in the kernel
sources, IOV_MAX in POSIX).  Because of this, on some host adapters
requests with many iovecs are rejected with -EINVAL by the
io_submit() or readv()/writev() system calls.

In fact, the same limit applies to SG_IO as well.  To fix both the
EINVAL and the possible performance issues from using fewer iovecs
than allowed by Linux (some HBAs have max_segments as low as 128),
introduce a separate entry in BlockLimits to hold the max_segments
value from sysfs.  This new limit is used only for SG_IO and clamped
to bs->bl.max_iov anyway, just like max_hw_transfer is clamped to
bs->bl.max_transfer.

Reported-by: Halil Pasic <pasic@linux.ibm.com>
Cc: Hanna Reitz <hreitz@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: qemu-block@nongnu.org
Cc: qemu-stable@nongnu.org
Fixes: 18473467d5 ("file-posix: try BLKSECTGET on block devices too, do not round to power of 2", 2021-06-25)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210923130436.1187591-1-pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Paolo Bonzini 2021-09-23 09:04:36 -04:00 • committed by Kevin Wolf
parent d318fc20b2
commit cc07162953
6 changed files with 17 additions and 2 deletions
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7
include/block/block_int.h
View file

@ -718,6 +718,13 @@ typedef struct BlockLimits {
*/
uint64_t max_hw_transfer;
/* Maximal number of scatter/gather elements allowed by the hardware.
* Applies whenever transfers to the device bypass the kernel I/O
* scheduler, for example with SG_IO. If larger than max_iov
* or if zero, blk_get_max_hw_iov will fall back to max_iov.
*/
int max_hw_iov;
/* memory alignment, in bytes so that no bounce buffer is needed */
size_t min_mem_alignment;