| Message ID | 1458577386-9984-2-git-send-email-alex.bennee@linaro.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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); > } > }
Paolo Bonzini <pbonzini@redhat.com> 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 <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); >> } >> } -- Alex Bennée
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); } }
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. 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(-)