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xen-devel
Re: [Xen-devel] [PATCH] x86/hvm/pmtimer: improving scalability of virtua
It can be as follows:
@@ -90,8 +90,8 @@ static void pmt_update_time(PMTState *s)
/* Update the timer */
curr_gtime = hvm_get_guest_time(s->vcpu);
- s->pm.tmr_val += ((curr_gtime - s->last_gtime) * s->scale) >> 32;
- s->pm.tmr_val &= TMR_VAL_MASK;
+ *(volatile uint32_t *)&s->pm.tmr_val = (s->pm.tmr_val +
+ (((curr_gtime - s->last_gtime) * s->scale) >> 32)) &
TMR_VAL_MASK;
s->last_gtime = curr_gtime;
/* If the counter's MSB has changed, set the status bit */
but it seems gcc (gcc 4.4.4) has omitted the statement s->pm.tmr_val
&= TMR_VAL_MASK;
in the object file
在 2010-11-17，下午2:47， Tim Deegan 写道：
At 21:39 +0000 on 17 Nov (1290029945), Song Xiang wrote:
diff --git a/xen/arch/x86/hvm/pmtimer.c b/xen/arch/x86/hvm/pmtimer.c
index 48fe26a..587944f 100644
--- a/xen/arch/x86/hvm/pmtimer.c
+++ b/xen/arch/x86/hvm/pmtimer.c
@@ -90,7 +90,7 @@ static void pmt_update_time(PMTState *s)
/* Update the timer */
curr_gtime = hvm_get_guest_time(s->vcpu);
- s->pm.tmr_val += ((curr_gtime - s->last_gtime) * s->scale) >>
32;
+ *(volatile uint32_t *)&s->pm.tmr_val = s->pm.tmr_val +
(((curr_gtime - s->last_gtime) * s->scale) >> 32);
s->pm.tmr_val &= TMR_VAL_MASK;
s->last_gtime = curr_gtime;
That doesn't make it an atomic update! The variable is still written
to twice. :) You need to calculate the new tmr_val in a scratch
variable, and then write it back once at the end of the function.
(And no 'volatile' wll be necessary).
Cheers,
Tim.
@@ -206,10 +206,19 @@ 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 acquired the PMTState lock then update the time
+ * else other vcpu is updating it ,it should be up to date.
+ */
+ if (spin_trylock(&s->lock)) {
+ pmt_update_time(s);
+ *val = s->pm.tmr_val;
+ spin_unlock(&s->lock);
+ }
+ else {
+ spin_barrier(&s->lock);
+ *val = s->pm.tmr_val;
+ }
return X86EMUL_OKAY;
}
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--
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd. (Company #02937203, SL9 0BG)
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