From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:58036) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ai3u1-0006Ep-Av for qemu-devel@nongnu.org; Mon, 21 Mar 2016 13:46:11 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ai3ty-0005ym-Kw for qemu-devel@nongnu.org; Mon, 21 Mar 2016 13:46:05 -0400 Received: from mail-wm0-x235.google.com ([2a00:1450:400c:c09::235]:37396) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ai3ty-0005yd-AK for qemu-devel@nongnu.org; Mon, 21 Mar 2016 13:46:02 -0400 Received: by mail-wm0-x235.google.com with SMTP id p65so131693967wmp.0 for ; Mon, 21 Mar 2016 10:46:02 -0700 (PDT) References: <1458577386-9984-1-git-send-email-alex.bennee@linaro.org> <1458577386-9984-2-git-send-email-alex.bennee@linaro.org> <56F0209E.6090706@redhat.com> From: Alex =?utf-8?Q?Benn=C3=A9e?= In-reply-to: <56F0209E.6090706@redhat.com> Date: Mon, 21 Mar 2016 17:45:59 +0000 Message-ID: <87mvprsp0o.fsf@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Subject: Re: [Qemu-devel] [PATCH v1 1/2] cpus: don't use atomic_read for vm_clock_warp_start List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: peter.maydell@linaro.org, sbruno@freebsd.org, Richard Henderson , qemu-devel@nongnu.org, Peter Crosthwaite Paolo Bonzini writes: > 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_*. I see you comment on the other thread passed the posting. I'll respin. > > Paolo > >> All other references to vm_clock_warp_start are already protected by the >> seqlock. >> >> Signed-off-by: Alex Bennée >> --- >> 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); >> } >> } -- Alex Bennée