From cc071629539dc1f303175a7e2d4ab854c0a8b20f Mon Sep 17 00:00:00 2001

From: Paolo Bonzini <pbonzini@redhat.com>

Date: Thu, 23 Sep 2021 09:04:36 -0400

Subject: [PATCH] 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>

---

 block/block-backend.c          | 6 ++++++

 block/file-posix.c             | 2 +-

 block/io.c                     | 1 +

 hw/scsi/scsi-generic.c         | 2 +-

 include/block/block_int.h      | 7 +++++++

 include/sysemu/block-backend.h | 1 +

 6 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/block/block-backend.c b/block/block-backend.c

index 6140d133e2..ba2b5ebb10 100644

--- a/block/block-backend.c

+++ b/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;

diff --git a/block/file-posix.c b/block/file-posix.c

index c62e42743d..53be0bdc1b 100644

--- a/block/file-posix.c

+++ b/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;

         }

     }

 }

diff --git a/block/io.c b/block/io.c

index 18d345a87a..bb0a254def 100644

--- a/block/io.c

+++ b/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 {

diff --git a/hw/scsi/scsi-generic.c b/hw/scsi/scsi-generic.c

index 665baf900e..0306ccc7b1 100644

--- a/hw/scsi/scsi-generic.c

+++ b/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)

diff --git a/include/block/block_int.h b/include/block/block_int.h

index ffe86068d4..f4c75e8ba9 100644

--- a/include/block/block_int.h

+++ b/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;

diff --git a/include/sysemu/block-backend.h b/include/sysemu/block-backend.h

index 29d4fdbf63..82bae55161 100644

--- a/include/sysemu/block-backend.h

+++ b/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);

-- 

2.33.1