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<[email protected]>

Index: qemu/block.c
===================================================================

block.c +4 -0

@@ -1689,6 +1689,10 @@ static int multiwrite_merge(BlockDriverS
              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));

Index: qemu/qemu-common.h
===================================================================

qemu-common.h +4 -0

@@ -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

Quoted 8 lines · Christoph Hellwig

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.

Quoted 2 lines

Signed-off-by Christoph Hellwig<[email protected]>

Applied. Thanks.