On 23/04/2018 10:45, Markus Armbruster wrote:
> When resume of a stopped guest immediately runs into block device
> errors, the BLOCK_IO_ERROR event is sent before the RESUME event.
>
> Reproducer:
>
> 1. Create a scratch image
> $ dd if=/dev/zero of=scratch.img bs=1M count=100
>
> Size doesn't actually matter.
>
> 2. Prepare blkdebug configuration:
>
> $ cat >blkdebug.conf <<EOF
> [inject-error]
> event = "write_aio"
> errno = "5"
> EOF
>
> Note that errno 5 is EIO.
>
> 3. Run a guest with an additional scratch disk, i.e. with additional
> arguments
> -drive
> if=none,id=scratch-drive,format=raw,werror=stop,file=blkdebug:blkdebug.conf:scratch.img
> -device virtio-blk-pci,id=scratch,drive=scratch-drive
>
> The blkdebug part makes all writes to the scratch drive fail with
> EIO. The werror=stop pauses the guest on write errors.
>
> 4. Connect to the QMP socket e.g. like this:
> $ socat UNIX:/your/qmp/socket
> READLINE,history=$HOME/.qmp_history,prompt='QMP> '
>
> Issue QMP command 'qmp_capabilities':
> QMP> { "execute": "qmp_capabilities" }
>
> 5. Boot the guest.
>
> 6. In the guest, write to the scratch disk, e.g. like this:
>
> # dd if=/dev/zero of=/dev/vdb count=1
>
> Do double-check the device specified with of= is actually the
> scratch device!
>
> 7. Issue QMP command 'cont':
> QMP> { "execute": "cont" }
>
> After step 6, I get a BLOCK_IO_ERROR event followed by a STOP event. Good.
>
> After step 7, I get BLOCK_IO_ERROR, then RESUME, then STOP. Not so
> good; I'd expect RESUME, then BLOCK_IO_ERROR, then STOP.
>
> The funny event order confuses libvirt: virsh -r domstate DOMAIN
> --reason reports "paused (unknown)" rather than "paused (I/O error)".
>
> The culprit is vm_prepare_start().
>
> /* Ensure that a STOP/RESUME pair of events is emitted if a
> * vmstop request was pending. The BLOCK_IO_ERROR event, for
> * example, according to documentation is always followed by
> * the STOP event.
> */
> if (runstate_is_running()) {
> qapi_event_send_stop(&error_abort);
> res = -1;
> } else {
> replay_enable_events();
> cpu_enable_ticks();
> runstate_set(RUN_STATE_RUNNING);
> vm_state_notify(1, RUN_STATE_RUNNING);
> }
>
> /* We are sending this now, but the CPUs will be resumed shortly later */
> qapi_event_send_resume(&error_abort);
> return res;
>
> When resuming a stopped guest, we take the else branch before we get
> to sending RESUME. vm_state_notify() runs virtio_vmstate_change(),
> among other things. This restarts I/O, triggering the BLOCK_IO_ERROR
> event.
>
> Reshuffle vm_prepare_start() to send the RESUME event earlier.
>
> Fixes RHBZ 1566153.
>
> Cc: Paolo Bonzini <pbonz...@redhat.com>
> Signed-off-by: Markus Armbruster <arm...@redhat.com>
> ---
> cpus.c | 16 ++++++++--------
> 1 file changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/cpus.c b/cpus.c
> index 38eba8bff3..398392bc3a 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -2043,7 +2043,6 @@ int vm_stop(RunState state)
> int vm_prepare_start(void)
> {
> RunState requested;
> - int res = 0;
>
> qemu_vmstop_requested(&requested);
> if (runstate_is_running() && requested == RUN_STATE__MAX) {
> @@ -2057,17 +2056,18 @@ int vm_prepare_start(void)
> */
> if (runstate_is_running()) {
> qapi_event_send_stop(&error_abort);
> - res = -1;
> - } else {
> - replay_enable_events();
> - cpu_enable_ticks();
> - runstate_set(RUN_STATE_RUNNING);
> - vm_state_notify(1, RUN_STATE_RUNNING);
> + qapi_event_send_resume(&error_abort);
> + return -1;
> }
>
> /* We are sending this now, but the CPUs will be resumed shortly later */
> qapi_event_send_resume(&error_abort);
> - return res;
> +
> + replay_enable_events();
> + cpu_enable_ticks();
> + runstate_set(RUN_STATE_RUNNING);
> + vm_state_notify(1, RUN_STATE_RUNNING);
> + return 0;
> }
>
> void vm_start(void)
>
Queued, thanks.
Paolo
