Re: [Qemu-devel] [PATCH v1 1/2] cpus: don't use atomic_read for vm_clock_warp_start

[Paolo Bonzini <pbonzini@redhat.com>]

On 21/03/2016 17:23, Alex Bennée wrote:
> As vm_clock_warp_start is a 64 bit value this causes problems for the
> compiler trying to come up with a suitable atomic operation on 32 bit
> hosts. The variable documentation says this is meant to be protected by
> vm_clock_seqlock so lets just move the code into that section.

This neglects the fact that there is a comment explaining the code, so

    Because the variable is protected by vm_clock_seqlock, we check its
    value inside a seqlock critical section.

is better.  In addition, I would prefer if you used seqlock_read_*.

Paolo

> All other references to vm_clock_warp_start are already protected by the
> seqlock.
> 
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> ---
>  cpus.c | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/cpus.c b/cpus.c
> index 23cf7aa..2fd5381 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -338,15 +338,10 @@ static int64_t qemu_icount_round(int64_t count)
>  
>  static void icount_warp_rt(void)
>  {
> -    /* The icount_warp_timer is rescheduled soon after vm_clock_warp_start
> -     * changes from -1 to another value, so the race here is okay.
> -     */
> -    if (atomic_read(&vm_clock_warp_start) == -1) {
> -        return;
> -    }
> +    bool check_clock = false;
>  
>      seqlock_write_lock(&timers_state.vm_clock_seqlock);
> -    if (runstate_is_running()) {
> +    if (vm_clock_warp_start != -1 && runstate_is_running()) {
>          int64_t clock = REPLAY_CLOCK(REPLAY_CLOCK_VIRTUAL_RT,
>                                       cpu_get_clock_locked());
>          int64_t warp_delta;
> @@ -362,11 +357,12 @@ static void icount_warp_rt(void)
>              warp_delta = MIN(warp_delta, delta);
>          }
>          timers_state.qemu_icount_bias += warp_delta;
> +        check_clock = true;
>      }
>      vm_clock_warp_start = -1;
>      seqlock_write_unlock(&timers_state.vm_clock_seqlock);
>  
> -    if (qemu_clock_expired(QEMU_CLOCK_VIRTUAL)) {
> +    if (check_clock && qemu_clock_expired(QEMU_CLOCK_VIRTUAL)) {
>          qemu_clock_notify(QEMU_CLOCK_VIRTUAL);
>      }
>  }