On 02/07/2011 10:00 AM, Jan Kiszka wrote:
On 2011-02-07 15:11, Zachary Amsden wrote:
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.
Hmm, is a cpu hotplug notifier allowed to run sleepy code? Can't
imagine. So we already have a strong reason to convert kvm_lock to a
raw_spinlock which obsoletes the above workaround.
I don't know as it is allowed to sleep, it doesn't call any sleeping
functions to my knowledge. What worries me in the RT case is that the
spinlock acquired for hardware_enable might be preempted and run on
another CPU, which obviously isn't what you want.
Zach
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