12.04.2024 18:25, Eric Blake wrote:
On Fri, Apr 12, 2024 at 10:06:17AM +0200, Stefan Fritsch wrote:
Commit 99868af3d0 changed the hardcoded constant BDRV_SECTOR_SIZE to a
dynamic field 'align' but introduced a bug. qemu_iovec_discard_back()
is now passed the wanted iov length instead of the actually required
amount that should be removed from the end of the iov.
The bug can likely only be hit in uncommon configurations, e.g. with
icount enabled or when reading from disk directly to device memory.
Hi!
Has this change (proposed for 9.0) been forgotten or is it not needed
anymore?
Thanks,
/mjt
Fixes: 99868af3d0a75cf6 ("dma-helpers: explicitly pass alignment into DMA
helpers")
Signed-off-by: Stefan Fritsch <s...@sfritsch.de>
---
system/dma-helpers.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
Wow, that bug has been latent for a while (2016, v2.8.0). As such, I
don't think it's worth holding up 9.0 for, but it is definitely worth
cc'ing qemu-stable (done now).
diff --git a/system/dma-helpers.c b/system/dma-helpers.c
index 9b221cf94e..c9677fd39b 100644
--- a/system/dma-helpers.c
+++ b/system/dma-helpers.c
@@ -174,8 +174,7 @@ static void dma_blk_cb(void *opaque, int ret)
}
if (!QEMU_IS_ALIGNED(dbs->iov.size, dbs->align)) {
- qemu_iovec_discard_back(&dbs->iov,
- QEMU_ALIGN_DOWN(dbs->iov.size, dbs->align));
+ qemu_iovec_discard_back(&dbs->iov, dbs->iov.size % dbs->align);
Before the regression, it was:
- if (dbs->iov.size & ~BDRV_SECTOR_MASK) {
- qemu_iovec_discard_back(&dbs->iov, dbs->iov.size & ~BDRV_SECTOR_MASK);
+ if (!QEMU_IS_ALIGNED(dbs->iov.size, dbs->align)) {
+ qemu_iovec_discard_back(&dbs->iov,
If dbs->align is always a power of two, we can use '& (dbs->align -
1)' to avoid a hardware division, to match the original code; bug
QEMU_IS_ALIGNED does not require a power of two, so your choice of '%
dbs->align' seems reasonable.
Reviewed-by: Eric Blake <ebl...@redhat.com>
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