When the guest sees a higher alignment than the host, writes may be
done in multiple steps. So, reads have to be serialized against
overlapping writes, so that the writes look atomic to the guest.
This is true even when O_DIRECT is not in use.
Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
---
block.c | 22 ++++++++++++++++++++++
1 files changed, 22 insertions(+), 0 deletions(-)
diff --git a/block.c b/block.c
index c78ca47..683d4a3 100644
--- a/block.c
+++ b/block.c
@@ -1613,6 +1613,16 @@ static int coroutine_fn
bdrv_co_do_readv(BlockDriverState *bs,
get_cluster_size(bs), false);
}
+ /* When the guest sees a higher alignment than the host, writes may be
+ * done in multiple steps. So, reads have to be serialized against
+ * overlapping writes, so that the writes look atomic to the guest,
+ * even when O_DIRECT is not in use.
+ */
+ if (bs->guest_block_size > bs->host_block_size) {
+ wait_for_overlapping_requests(bs, sector_num, nb_sectors,
+ bs->guest_block_size, true);
+ }
+
tracked_request_begin(&req, bs, sector_num, nb_sectors, false);
if (flags & BDRV_REQ_COPY_ON_READ) {
@@ -3629,6 +3639,18 @@ BlockDriverAIOCB *bdrv_aio_ioctl(BlockDriverState *bs,
void bdrv_set_guest_block_size(BlockDriverState *bs, int align)
{
bs->guest_block_size = align;
+ if ((bs->open_flags & BDRV_O_RDWR) &&
+ bs->host_block_size < bs->guest_block_size) {
+ error_report("Host block size is %d, guest block size is %d. Due to
partially\n"
+ "written sectors, power failures may cause data
corruption.%s",
+ bs->host_block_size, bs->guest_block_size,
+
+ /* The host block size might not be detected correctly if
+ * the guest is not using O_DIRECT. */
+ (bs->open_flags & BDRV_O_NOCACHE) ? "" : "\n"
+ "If you think this message is wrong, start the guest with
cache=none\n"
+ "and see if it disappears.");
+ }
}
void *qemu_blockalign(BlockDriverState *bs, size_t size)
--
1.7.7.6
