Re: [PATCH 3/3 V2] pvclock: Get rid of __pvclock_read_cycles in function pvclock_read_flags

[Peter Zijlstra]

On Thu, Jun 09, 2016 at 02:16:03PM +0200, Peter Zijlstra wrote:
> What's wrong with:
> 
>       u8 flags = READ_ONCE(src->flags);
> 
> ?
> 
> (and have the flags store be done using WRITE_ONCE() of course).
> 
> Sure, if your total state is larger than one word you need the
> seqcount for integrity, but reading _one_ byte, shees.

Something like so; although given that I've never seen this code before
I can easily have missed an update site.

diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
index 99bfc025111d..7a4bf59aa929 100644
--- a/arch/x86/kernel/pvclock.c
+++ b/arch/x86/kernel/pvclock.c
@@ -60,15 +60,7 @@ void pvclock_resume(void)
 
 u8 pvclock_read_flags(struct pvclock_vcpu_time_info *src)
 {
-       unsigned version;
-       cycle_t ret;
-       u8 flags;
-
-       do {
-               version = __pvclock_read_cycles(src, &ret, &flags);
-       } while ((src->version & 1) || version != src->version);
-
-       return flags & valid_flags;
+       return READ_ONCE(src->flags) & valid_flags;
 }
 
 cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
@@ -83,7 +75,8 @@ cycle_t pvclock_clocksource_read(struct 
pvclock_vcpu_time_info *src)
        } while ((src->version & 1) || version != src->version);
 
        if (unlikely((flags & PVCLOCK_GUEST_STOPPED) != 0)) {
-               src->flags &= ~PVCLOCK_GUEST_STOPPED;
+               flags &= ~PVCLOCK_GUEST_STOPPED;
+               WRITE_ONCE(src->flags, flags);
                pvclock_touch_watchdogs();
        }
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1ba3b7d3cae9..54458acce4f5 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1832,7 +1832,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
        if (use_master_clock)
                pvclock_flags |= PVCLOCK_TSC_STABLE_BIT;
 
-       vcpu->hv_clock.flags = pvclock_flags;
+       WRITE_ONCE(vcpu->hv_clock.flags, pvclock_flags);
 
        trace_kvm_pvclock_update(v->vcpu_id, &vcpu->hv_clock);