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Re: [Qemu-devel] [PATCH v1 1/2] cpus: don't use atomic_read for vm_clock
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH v1 1/2] cpus: don't use atomic_read for vm_clock_warp_start |
Date: |
Mon, 21 Mar 2016 17:26:06 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 |
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 <address@hidden>
> ---
> 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);
> }
> }