linux-kernel - Re: [PATCH] KVM: x86: Convert tsc_write_lock to raw

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Message-ID: <4D4FD8EE.6040009@siemens.com>
Date:	Mon, 07 Feb 2011 12:35:10 +0100
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Zachary Amsden <zamsden@...hat.com>
CC:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	kvm <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: Convert tsc_write_lock to raw_spinlock

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

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux
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