On Thu, 23 Sep 2021 09:04:36 -0400 Paolo Bonzini <pbonz...@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 <pa...@linux.ibm.com> > Cc: Hanna Reitz <hre...@redhat.com> > Cc: Kevin Wolf <kw...@redhat.com> > Cc: qemu-bl...@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 <pbonz...@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);