Am 10.03.23 um 16:13 schrieb Paolo Bonzini: > On Fri, Mar 10, 2023 at 3:25 PM Kevin Wolf <kw...@redhat.com> wrote: >>> 1. The TRIM operation should be completed on the IDE level before >>> draining ends. >>> 2. Block layer requests issued after draining has begun are queued. >>> >>> To me, the conclusion seems to be: >>> Issue all block layer requests belonging to the IDE TRIM operation up >>> front. >>> >>> The other alternative I see is to break assumption 2, introduce a way >>> to not queue certain requests while drained, and use it for the >>> recursive requests issued by ide_issue_trim_cb. But not the initial >>> one, if that would defeat the purpose of request queuing. Of course >>> this can't be done if QEMU relies on the assumption in other places >>> already. >> >> I feel like this should be allowed because if anyone has exclusive >> access in this scenario, it's IDE, so it should be able to bypass the >> queuing. Of course, the queuing is still needed if someone else drained >> the backend, so we can't just make TRIM bypass it in general. And if you >> make it conditional on IDE being in blk_drain(), it already starts to >> become ugly again... >> >> So maybe the while loop is unavoidable. >> >> Hmm... But could ide_cancel_dma_sync() just directly use >> AIO_WAIT_WHILE(s->bus->dma->aiocb) instead of using blk_drain()? > > While that should work, it would not fix other uses of > bdrv_drain_all(), for example in softmmu/cpus.c. Stopping the device > model relies on those to run *until the device model has finished > submitting requests*. > > So I still think that this bug is a symptom of a problem in the design > of request queuing. > > In fact, shouldn't request queuing was enabled at the _end_ of > bdrv_drained_begin (once the BlockBackend has reached a quiescent > state on its own terms), rather than at the beginning (which leads to > deadlocks like this one)? > > blk->quiesce_counter becomes just a nesting counter for > drained_begin/end, with no uses outside, and blk_wait_while_drained > uses a new counter. Then you have something like this in > blk_root_drained_poll: > > if (blk->dev_ops && blk->dev_ops->drained_poll) { > busy = blk->dev_ops->drained_poll(blk->dev_opaque); > } > busy |= !!blk->in_flight; > if (!busy) { > qatomic_set(&blk->queue_requests, true); > } > return busy; > > And there's no need to touch IDE at all. > Couldn't this lead to scenarios where a busy or malicious guest, which continues to submit new requests, slows down draining or even prevents it from finishing?
Best Regards, Fiona