diff options
| -rw-r--r-- | xen/arch/x86/hvm/pmtimer.c | 34 |
1 files changed, 25 insertions, 9 deletions
diff --git a/xen/arch/x86/hvm/pmtimer.c b/xen/arch/x86/hvm/pmtimer.c index 2070525abe..01ae31d38d 100644 --- a/xen/arch/x86/hvm/pmtimer.c +++ b/xen/arch/x86/hvm/pmtimer.c @@ -88,7 +88,7 @@ void hvm_acpi_sleep_button(struct domain *d) static void pmt_update_time(PMTState *s) { uint64_t curr_gtime, tmp; - uint32_t msb = s->pm.tmr_val & TMR_VAL_MSB; + uint32_t tmr_val = s->pm.tmr_val, msb = tmr_val & TMR_VAL_MSB; ASSERT(spin_is_locked(&s->lock)); @@ -96,12 +96,15 @@ static void pmt_update_time(PMTState *s) curr_gtime = hvm_get_guest_time(s->vcpu); tmp = ((curr_gtime - s->last_gtime) * s->scale) + s->not_accounted; s->not_accounted = (uint32_t)tmp; - s->pm.tmr_val += tmp >> 32; - s->pm.tmr_val &= TMR_VAL_MASK; + tmr_val += tmp >> 32; + tmr_val &= TMR_VAL_MASK; s->last_gtime = curr_gtime; - + + /* Update timer value atomically wrt lock-free reads in handle_pmt_io(). */ + *(volatile uint32_t *)&s->pm.tmr_val = tmr_val; + /* If the counter's MSB has changed, set the status bit */ - if ( (s->pm.tmr_val & TMR_VAL_MSB) != msb ) + if ( (tmr_val & TMR_VAL_MSB) != msb ) { s->pm.pm1a_sts |= TMR_STS; pmt_update_sci(s); @@ -215,10 +218,23 @@ static int handle_pmt_io( if ( dir == IOREQ_READ ) { - spin_lock(&s->lock); - pmt_update_time(s); - *val = s->pm.tmr_val; - spin_unlock(&s->lock); + if ( spin_trylock(&s->lock) ) + { + /* We hold the lock: update timer value and return it. */ + pmt_update_time(s); + *val = s->pm.tmr_val; + spin_unlock(&s->lock); + } + else + { + /* + * Someone else is updating the timer: rather than do the work + * again ourselves, wait for them to finish and then steal their + * updated value with a lock-free atomic read. + */ + spin_barrier(&s->lock); + *val = *(volatile uint32_t *)&s->pm.tmr_val; + } return X86EMUL_OKAY; } |