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>

6 files changed, 25 insertions, 2 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)
 {
diff --git a/block/file-posix.c b/block/file-posix.c
index 6db690baf2..88e58d2863 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -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);
diff --git a/block/io.c b/block/io.c
index 323854d063..dd93364258 100644
--- a/block/io.c
+++ b/block/io.c
@@ -127,6 +127,8 @@ static void bdrv_merge_limits(BlockLimits *dst, const BlockLimits *src)
 {
     dst->opt_transfer = MAX(dst->opt_transfer, src->opt_transfer);
     dst->max_transfer = MIN_NON_ZERO(dst->max_transfer, src->max_transfer);
+    dst->max_hw_transfer = MIN_NON_ZERO(dst->max_hw_transfer,
+                                        src->max_hw_transfer);
     dst->opt_mem_alignment = MAX(dst->opt_mem_alignment,
                                  src->opt_mem_alignment);
     dst->min_mem_alignment = MAX(dst->min_mem_alignment,
diff --git a/hw/scsi/scsi-generic.c b/hw/scsi/scsi-generic.c
index b6c4143dc7..665baf900e 100644
--- a/hw/scsi/scsi-generic.c
+++ b/hw/scsi/scsi-generic.c
@@ -179,7 +179,7 @@ static int scsi_handle_inquiry_reply(SCSIGenericReq *r, SCSIDevice *s, int len)
         (r->req.cmd.buf[1] & 0x01)) {
         page = r->req.cmd.buf[2];
         if (page == 0xb0) {
-            uint32_t max_transfer = blk_get_max_transfer(s->conf.blk);
+            uint64_t max_transfer = blk_get_max_hw_transfer(s->conf.blk);
             uint32_t max_iov = blk_get_max_iov(s->conf.blk);
 
             assert(max_transfer);
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 057d88b1fc..f1a54db0f8 100644
--- a/include/block/block_int.h
+++ 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
+     * 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.
+     */
+    uint64_t max_hw_transfer;
+
     /* memory alignment, in bytes so that no bounce buffer is needed */
     size_t min_mem_alignment;
 
diff --git a/include/sysemu/block-backend.h b/include/sysemu/block-backend.h
index 5423e3d9c6..9ac5f7bbd3 100644
--- a/include/sysemu/block-backend.h
+++ b/include/sysemu/block-backend.h
@@ -208,6 +208,7 @@ void blk_eject(BlockBackend *blk, bool eject_flag);
 int blk_get_flags(BlockBackend *blk);
 uint32_t blk_get_request_alignment(BlockBackend *blk);
 uint32_t blk_get_max_transfer(BlockBackend *blk);
+uint64_t blk_get_max_hw_transfer(BlockBackend *blk);
 int blk_get_max_iov(BlockBackend *blk);
 void blk_set_guest_block_size(BlockBackend *blk, int align);
 void *blk_try_blockalign(BlockBackend *blk, size_t size);