block: avoid creating too large iovecs in multiwrite_merge

If we go over the maximum number of iovecs support by syscall we get
back EINVAL from the kernel which translate to I/O errors for the guest.

Add a MAX_IOV defintion for platforms that don't have it.  For now we use
the same 1024 define that's used on Linux and various other platforms,
but until the windows block backend implements some kind of vectored I/O
it doesn't matter.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

diff --git a/block.c b/block.c
index 9c106a8e4dc3d4a93011aeea2a5fe1decb74298a..1919d1973287e52570a3025449147ce3a6589d76 100644 (file)
--- a/block.c
+++ b/block.c
@@ -1691,6 +1691,10 @@ static int multiwrite_merge(BlockDriverState *bs, BlockRequest *reqs,
             merge = bs->drv->bdrv_merge_requests(bs, &reqs[outidx], &reqs[i]);
         }
 
+        if (reqs[outidx].qiov->niov + reqs[i].qiov->niov + 1 > IOV_MAX) {
+            merge = 0;
+        }
+
         if (merge) {
             size_t size;
             QEMUIOVector *qiov = qemu_mallocz(sizeof(*qiov));

diff --git a/qemu-common.h b/qemu-common.h
index ae4f23b8e866b058bb7c6027fdb3b22a0e2b62d7..b09f71757a3875468fd7a00a9d9a6234528c4568 100644 (file)
--- a/qemu-common.h
+++ b/qemu-common.h
@@ -54,6 +54,10 @@ struct iovec {
     void *iov_base;
     size_t iov_len;
 };
+/*
+ * Use the same value as Linux for now.
+ */
+#define IOV_MAX                1024
 #else
 #include <sys/uio.h>
 #endif