Re: [Qemu-devel] Block IO throttling: contractor wanted

[Stefan Hajnoczi]

On Tue, Feb 14, 2012 at 2:30 PM, Chris Webb <address@hidden> wrote:
> However, there are (known) run-time
> assertion failures with throttled IDE devices[1], which show up in qemu-kvm
> 1.0 and apparently also in qemu HEAD. We have also sometimes seen throttled
> VMs spinning unresponsively with 100% CPU on start-up, which may be related.
...
> [1] I can't immediately find the original reports in the archives, but I
> discussed this privately with Zhi Yong Wu and he had already had reports of
> the same issue. As a quick example, I can trigger an assertion failure in
> the IDE driver by turning on limits on a running guest doing heavy IO. I
> configure a guest with an IDE drive ide.0.0 and then do
>
>  block_set_io_throttle ide.0.0 100000000 0 0 1000 0 0
>
> Shortly afterwards, the qemu-kvm process exists with an assert():
>
> qemu-kvm: /home/root/packages/qemu-kvm-1.0/src-76ig7q/hw/ide/pci.c:313:
>  bmdma_cmd_writeb: Assertion `bm->bus->dma->aiocb == ((void *)0)' failed.
>
> i.e. bm->bus->dma->aiocb is not NULL after qemu_aio_flush() in
> bmdma_cmd_writeb in the IDE driver:
>
>  void bmdma_cmd_writeb(BMDMAState *bm, uint32_t val)
>  {
>  #ifdef DEBUG_IDE
>      printf("%s: 0x%08x\n", __func__, val);
>  #endif
>
>      /* 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) {
>                  qemu_aio_flush();
>                  assert(bm->bus->dma->aiocb == NULL);
>                  assert((bm->status & BM_STATUS_DMAING) == 0);
>              }
>          } else {
>              bm->cur_addr = bm->addr;
>              if (!(bm->status & BM_STATUS_DMAING)) {
>                  bm->status |= BM_STATUS_DMAING;
>                  /* start dma transfer if possible */
>                  if (bm->dma_cb)
>                      bm->dma_cb(bmdma_active_if(bm), 0);
>              }
>          }
>      }
>
>      bm->cmd = val & 0x09;
>  }
>
> (My uninformed guess is that this might be something to do with
> qemu_aio_flush() not being able to write out all the data because of the IO
> throttling?)

Thanks for the bug report.  This is an actively maintained part of the
codebase, so chances are good this can be fixed in a reasonable time
by the community.

Just wanted to share my thoughts in case no one else replies - Zhi
Yong and I are aware of this bug and there should be time to look into
it.

Stefan