the blk_drain_all() that is executed if the guest issues a DMA cancel
leads to a stuck main loop if the storage backend (e.g. a NFS share)
is unresponsive.
This scenario is a common case for CDROM images mounted from an
NFS share. In this case a broken NFS server can take down the
whole VM even if the mounted CDROM is not used and was just not
unmounted after usage.
This approach avoids the blk_drain_all for read-only media and
cancelles the AIO locally and makes the callback a NOP if the
original request is completed after the NFS share is responsive
again.
Signed-off-by: Peter Lieven <p...@kamp.de>
---
hw/ide/pci.c | 32 ++++++++++++++++++--------------
1 file changed, 18 insertions(+), 14 deletions(-)
diff --git a/hw/ide/pci.c b/hw/ide/pci.c
index d31ff88..a8b4175 100644
--- a/hw/ide/pci.c
+++ b/hw/ide/pci.c
@@ -240,21 +240,25 @@ void bmdma_cmd_writeb(BMDMAState *bm, uint32_t val)
/* Ignore writes to SSBM if it keeps the old value */
if ((val & BM_CMD_START) != (bm->cmd & BM_CMD_START)) {
if (!(val & BM_CMD_START)) {
- /*
- * We can't cancel Scatter Gather DMA in the middle of the
- * operation or a partial (not full) DMA transfer would reach
- * the storage so we wait for completion instead (we beahve
- * like if the DMA was completed by the time the guest trying
- * to cancel dma with bmdma_cmd_writeb with BM_CMD_START not
- * set).
- *
- * In the future we'll be able to safely cancel the I/O if the
- * whole DMA operation will be submitted to disk with a single
- * aio operation with preadv/pwritev.
- */
if (bm->bus->dma->aiocb) {
- blk_drain_all();
- assert(bm->bus->dma->aiocb == NULL);
+ if (!bdrv_is_read_only(bm->bus->dma->aiocb->bs)) {
+ /* We can't cancel Scatter Gather DMA in the middle of the
+ * operation or a partial (not full) DMA transfer would
+ * reach the storage so we wait for completion instead
+ * (we beahve like if the DMA was completed by the time the
+ * guest trying to cancel dma with bmdma_cmd_writeb with
+ * BM_CMD_START not set). */
+ blk_drain_all();
+ assert(bm->bus->dma->aiocb == NULL);
+ } else {
+ /* On a read-only device (e.g. CDROM) we can't cause incon-
+ * sistencies and thus cancel the AIOCB locally and avoid
+ * to be called back later if the original request is
+ * completed. */
+ BlockAIOCB *aiocb = bm->bus->dma->aiocb;
+ aiocb->cb(aiocb->opaque, -ECANCELED);
+ aiocb->cb = NULL;
+ }
}
bm->status &= ~BM_STATUS_DMAING;
} else {
--
1.9.1
