On Wed, Mar 18, 2015 at 02:49:28PM +0800, Fam Zheng wrote:
> +static coroutine_fn int null_co_common(BlockDriverState *bs)
> +{
> +    BDRVNullState *s = bs->opaque;
> +
> +    /* Only emulate latency after vcpu is running. */
> +    if (s->latency_ns && qemu_clock_get_us(QEMU_CLOCK_VIRTUAL) > 0) {

This seems a little hacky.  There is runstate_is_running(), which
doesn't depend on the CPU clock offset being 0 in qemu_clock_get_us().