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[Qemu-devel] [PATCH for-2.5 1/4] block: add BlockLimits.max_iov field
From: |
Stefan Hajnoczi |
Subject: |
[Qemu-devel] [PATCH for-2.5 1/4] block: add BlockLimits.max_iov field |
Date: |
Wed, 8 Jul 2015 16:30:59 +0100 |
The maximum number of struct iovec elements depends on the
BlockDriverState. The raw-posix protocol has a maximum of IOV_MAX but
others could have different values.
Instead of assuming raw-posix and hardcoding IOV_MAX in several places,
put the limit into BlockLimits.
Cc: Peter Lieven <address@hidden>
Suggested-by: Kevin Wolf <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>
---
Peter Lieven: I think the SCSI LUN level does not have a maximum
scatter-gather segments constraint. That is probably only at the HBA
level. CCed you anyway in case you think block/iscsi.c should set the
max_iov field.
Kevin: The default is now INT_MAX. This means non-raw-posix users will
now be able to merge more requests than before. They were limited to
IOV_MAX previously. This could expose limits in other BlockDrivers
which we weren't aware of...
---
block/io.c | 3 +++
block/raw-posix.c | 1 +
include/block/block_int.h | 3 +++
3 files changed, 7 insertions(+)
diff --git a/block/io.c b/block/io.c
index e295992..6750de6 100644
--- a/block/io.c
+++ b/block/io.c
@@ -165,9 +165,11 @@ void bdrv_refresh_limits(BlockDriverState *bs, Error
**errp)
bs->bl.max_transfer_length = bs->file->bl.max_transfer_length;
bs->bl.min_mem_alignment = bs->file->bl.min_mem_alignment;
bs->bl.opt_mem_alignment = bs->file->bl.opt_mem_alignment;
+ bs->bl.max_iov = bs->file->bl.max_iov;
} else {
bs->bl.min_mem_alignment = 512;
bs->bl.opt_mem_alignment = getpagesize();
+ bs->bl.max_iov = INT_MAX;
}
if (bs->backing_hd) {
@@ -188,6 +190,7 @@ void bdrv_refresh_limits(BlockDriverState *bs, Error **errp)
bs->bl.min_mem_alignment =
MAX(bs->bl.min_mem_alignment,
bs->backing_hd->bl.min_mem_alignment);
+ bs->bl.max_iov = MIN(bs->bl.max_iov, bs->backing_hd->bl.max_iov);
}
/* Then let the driver override it */
diff --git a/block/raw-posix.c b/block/raw-posix.c
index cbe6574..faa6ae0 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -735,6 +735,7 @@ static void raw_refresh_limits(BlockDriverState *bs, Error
**errp)
raw_probe_alignment(bs, s->fd, errp);
bs->bl.min_mem_alignment = s->buf_align;
bs->bl.opt_mem_alignment = MAX(s->buf_align, getpagesize());
+ bs->bl.max_iov = IOV_MAX; /* limit comes from preadv()/pwritev()/etc */
}
static int check_for_dasd(int fd)
diff --git a/include/block/block_int.h b/include/block/block_int.h
index b0476fc..767e83d 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -315,6 +315,9 @@ typedef struct BlockLimits {
/* memory alignment for bounce buffer */
size_t opt_mem_alignment;
+
+ /* maximum number of iovec elements */
+ int max_iov;
} BlockLimits;
typedef struct BdrvOpBlocker BdrvOpBlocker;
--
2.4.3