Re: [PATCH] block: introduce max_hw_iov for use in scsi-generic

[Halil Pasic <pasic@linux.ibm.com>]

On Thu, 23 Sep 2021 09:04:36 -0400
Paolo Bonzini <pbonzini@redhat.com> wrote:

> 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.

Doesn't this patch render bs->bl.max_iov a constant?

$ git grep -p -e 'bl\(.\|->\)max_iov'
block/block-backend.c=int blk_get_max_iov(BlockBackend *blk)
block/block-backend.c:    return blk->root->bs->bl.max_iov;
block/file-posix.c=static void raw_refresh_limits(BlockDriverState *bs, Error **errp)
block/file-posix.c:            bs->bl.max_iov = ret;
block/io.c=void bdrv_refresh_limits(BlockDriverState *bs, Transaction *tran, Error **errp)
block/io.c:        bs->bl.max_iov = IOV_MAX;
block/mirror.c=static int coroutine_fn mirror_run(Job *job, Error **errp)
block/mirror.c:    s->max_iov = MIN(bs->bl.max_iov, target_bs->bl.max_iov);

Can't we use some of the established constants instead of hard coding a
qemu specific IOV_MAX?

POSIX.1 seems to guarantee the availability of IOV_MAX in <limits.h>
according to: https://man7.org/linux/man-pages/man2/readv.2.html
and <sys/uio.h> may have UIO_MAXIOV defined.

> 
> 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
> 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>
> ---
>  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 cb9bffe047..1567edb3d5 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 a19942718b..f38e7f81d8 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 f1a54db0f8..c31cbd034a 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -702,6 +702,13 @@ 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);