block: Switch to host monotonic clock for IO throttling

Enterprise / Virtualization / QEMU - Fam Zheng [redhat.com] - 28 April 2015 08:36 UTC

Currently, throttle timers won't make any progress when VCPU is not running, which would stall the request queue in utils, qtest, vm suspending, and live migration, without special handling.

Block jobs are confusingly inconsistent between with and without throttling: if user sets a bps limit, stops the vm, then start a block job, the block job will not make any progress; in contrary, if user unsets the bps limit, or if it's not set, the block job will run normally.

After this patch, with the host clock, even if the VCPUs are stopped, the throttle queues will be processed.

This patch also enables potential to add throttle to bdrv_drain_all. Currently all requests are drained immediately. In other words whenever it is called, IO throttling goes ineffective (examples: system reset, migration and many block job operations.). This is a loophole that guest could exploit. If we use the host clock, we can later just trust the nested poll. This could be done on top.

Note that for qemu-iotests case 093, which uses qtest, we still keep vm clock so the script can control the clock stepping in order to be deterministic.

de50a20 block: Switch to host monotonic clock for IO throttling
block.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

Upstream: git.qemu.org

  • Share