[PATCH 2/2] ide: Explicitly poll for BHs on cancel

[Lukas Straub]

When we still have an AIOCB registered for DMA operations, we try to
settle the respective operation by draining the BlockBackend associated
with the IDE device.

However, this assumes that every DMA operation is associated with some
I/O operation on the BlockBackend, and so settling the latter will
settle the former.  That is not the case; for example, the guest is free
to issue a zero-length TRIM operation that will not result in any I/O
operation forwarded to the BlockBackend.  In such a case, blk_drain()
will be a no-op if no other operations are in flight.

It is clear that if blk_drain() is a no-op, the value of
s->bus->dma->aiocb will not change between checking it in the `if`
condition and asserting that it is NULL after blk_drain().

To settle the DMA operation, we will thus need to explicitly invoke
aio_poll() ourselves, which will run any outstanding BHs (like
ide_trim_bh_cb()), until s->bus->dma->aiocb is NULL.  To stop this from
being an infinite loop, assert that we made progress with every
aio_poll() call (i.e., invoked some BH).

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2029980
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Lukas Straub <lukasstraub2@web.de>
---
 hw/ide/core.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/hw/ide/core.c b/hw/ide/core.c
index d172e70f1e..a5fd89ebdd 100644
--- a/hw/ide/core.c
+++ b/hw/ide/core.c
@@ -736,7 +736,17 @@ void ide_cancel_dma_sync(IDEState *s)
     if (s->bus->dma->aiocb) {
         trace_ide_cancel_dma_sync_remaining();
         blk_drain(s->blk);
-        assert(s->bus->dma->aiocb == NULL);
+
+        /*
+         * Wait for potentially still-scheduled BHs, like ide_trim_bh_cb()
+         * (blk_drain() will only poll if there are in-flight requests on the
+         * BlockBackend, which there may not necessarily be, e.g. when the
+         * guest has issued a zero-length TRIM request)
+         */
+        while (s->bus->dma->aiocb) {
+            bool progress = aio_poll(qemu_get_aio_context(), true);
+            assert(progress);
+        }
     }
 }
 
-- 
2.39.2

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