Messages in this thread | | | Date | Mon, 07 Feb 2011 09:11:35 -0500 | From | Zachary Amsden <> | Subject | Re: [PATCH] KVM: x86: Convert tsc_write_lock to raw_spinlock |
| |
On 02/07/2011 06:35 AM, Jan Kiszka wrote: > On 2011-02-04 22:03, Zachary Amsden wrote: > >> On 02/04/2011 04:49 AM, Jan Kiszka wrote: >> >>> Code under this lock requires non-preemptibility. Ensure this also over >>> -rt by converting it to raw spinlock. >>> >>> >> Oh dear, I had forgotten about that. I believe kvm_lock might have the >> same assumption in a few places regarding clock. >> > I only found a problematic section in kvmclock_cpufreq_notifier. Didn't > see this during my tests as I have CPUFREQ disabled in my .config. > > We may need something like this as converting kvm_lock would likely be > overkill: > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 36f54fb..971ee0d 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -4530,7 +4530,7 @@ static int kvmclock_cpufreq_notifier(struct notifier_block *nb, unsigned long va > struct cpufreq_freqs *freq = data; > struct kvm *kvm; > struct kvm_vcpu *vcpu; > - int i, send_ipi = 0; > + int i, me, send_ipi = 0; > > /* > * We allow guests to temporarily run on slowing clocks, > @@ -4583,9 +4583,11 @@ static int kvmclock_cpufreq_notifier(struct notifier_block *nb, unsigned long va > kvm_for_each_vcpu(i, vcpu, kvm) { > if (vcpu->cpu != freq->cpu) > continue; > + me = get_cpu(); > kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu); > - if (vcpu->cpu != smp_processor_id()) > + if (vcpu->cpu != me) > send_ipi = 1; > + put_cpu(); > } > } > spin_unlock(&kvm_lock); > > Jan > >
That looks like a good solution, and I do believe that is the only place the lock is used in that fashion - please add a comment though in the giant comment block above that preemption protection is needed for RT. Also, gcc should catch this, but moving the me variable into the kvm_for_each_vcpu loop should allow for better register allocation.
The only other thing I can think of is that RT lock preemption may break some of the CPU initialization semantics enforced by kvm_lock if you happen to get a hotplug event just as the module is loading. That should be rare, but if it is indeed a bug, it would be nice to fix, it would be a panic for sure not to initialize VMX.
Cheers,
Zach
| |