block: add max_hw_transfer to BlockLimits

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>

1 files changed, 13 insertions, 0 deletions

diff --git a/block/block-backend.c b/block/block-backend.c
index 6e37582740..deb55c272e 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -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)
 {