@@ -, +, @@ 
    block: introduce max_hw_iov for use in scsi-generic
    
    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>
--- a/block/block-backend.c	
+++ a/block/block-backend.c	
@@ -1986,6 +1986,12 @@ uint32_t blk_get_max_transfer(BlockBackend *blk)
     return ROUND_DOWN(max, blk_get_request_alignment(blk));
 }
 
+int blk_get_max_hw_iov(BlockBackend *blk)
+{
+    return MIN_NON_ZERO(blk->root->bs->bl.max_hw_iov,
+                        blk->root->bs->bl.max_iov);
+}
+
 int blk_get_max_iov(BlockBackend *blk)
 {
     return blk->root->bs->bl.max_iov;
--- a/block/file-posix.c	
+++ a/block/file-posix.c	
@@ -1273,7 +1273,7 @@ static void raw_refresh_limits(BlockDriverState *bs, Error **errp)
 
         ret = hdev_get_max_segments(s->fd, &st);
         if (ret > 0) {
-            bs->bl.max_iov = ret;
+            bs->bl.max_hw_iov = ret;
         }
     }
 }
--- a/block/io.c	
+++ a/block/io.c	
@@ -136,6 +136,7 @@ static void bdrv_merge_limits(BlockLimits *dst, const BlockLimits *src)
     dst->min_mem_alignment = MAX(dst->min_mem_alignment,
                                  src->min_mem_alignment);
     dst->max_iov = MIN_NON_ZERO(dst->max_iov, src->max_iov);
+    dst->max_hw_iov = MIN_NON_ZERO(dst->max_hw_iov, src->max_hw_iov);
 }
 
 typedef struct BdrvRefreshLimitsState {
--- a/hw/scsi/scsi-generic.c	
+++ a/hw/scsi/scsi-generic.c	
@@ -180,7 +180,7 @@ static int scsi_handle_inquiry_reply(SCSIGenericReq *r, SCSIDevice *s, int len)
         page = r->req.cmd.buf[2];
         if (page == 0xb0) {
             uint64_t max_transfer = blk_get_max_hw_transfer(s->conf.blk);
-            uint32_t max_iov = blk_get_max_iov(s->conf.blk);
+            uint32_t max_iov = blk_get_max_hw_iov(s->conf.blk);
 
             assert(max_transfer);
             max_transfer = MIN_NON_ZERO(max_transfer, max_iov * qemu_real_host_page_size)
--- a/include/block/block_int.h	
+++ a/include/block/block_int.h	
@@ -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;
 
--- a/include/sysemu/block-backend.h	
+++ a/include/sysemu/block-backend.h	
@@ -211,6 +211,7 @@ 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);
+int blk_get_max_hw_iov(BlockBackend *blk);
 void blk_set_guest_block_size(BlockBackend *blk, int align);
 void *blk_try_blockalign(BlockBackend *blk, size_t size);
 void *blk_blockalign(BlockBackend *blk, size_t size);