Re: [Qemu-devel] [PATCH 2/2] virtio-rng: stop virtqueue while the CPU is stopped

[Laurent Vivier <lvivier@redhat.com>]

On 11/04/2017 19:02, Stefan Hajnoczi wrote:
> On Tue, Apr 11, 2017 at 03:17:33PM +0200, Laurent Vivier wrote:
>> diff --git a/hw/virtio/virtio-rng.c b/hw/virtio/virtio-rng.c
>> index 9639f4e..d270d56 100644
>> --- a/hw/virtio/virtio-rng.c
>> +++ b/hw/virtio/virtio-rng.c
>> @@ -53,6 +53,15 @@ static void chr_read(void *opaque, const void *buf, size_t size)
>>          return;
>>      }
>>  
>> +    /* we can't modify the virtqueue until
>> +     * our state is fully synced
>> +     */
>> +
>> +    if (!runstate_check(RUN_STATE_RUNNING)) {
>> +        trace_virtio_rng_cpu_is_stopped(vrng);
>> +        return;
>> +    }
>> +
> 
> I'm concerned about what happens when the guest is stopped and resumed
> (e.g. 'stop' and 'cont' monitor commands).  Since we throw away the
> chr_read() callback the device will hang unless the guest kicks it
> again?
> 
> It's not clear to me that the rate limit timer will help us...

I think you're right (even if it seems hard to generate this case)

What is the best solution:

- re-arming the timer/the backend request by calling
virtio_rng_process() before the "return;"

or

- adding a vmstate change handler to call virtio_rng_process()?

Thanks,
Laurent