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Date: Tue, 5 May 2009 18:05:44 +0200
From: Bert Wesarg <bert.wesarg@...glemail.com>
To: Mark Langsdorf <mark.langsdorf@....com>
Cc: joerg.roedel@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][KVM] Add support for Pause Filtering to AMD SVM
On Tue, May 5, 2009 at 16:09, Mark Langsdorf <mark.langsdorf@....com> wrote:
> commit 6f15c833f56267baf5abdd0fbc90a81489573053
> Author: Mark Langsdorf <mlangsdo@...pnow.amd.com>
> Date: Mon May 4 15:02:38 2009 -0500
>
> New AMD processors will support the Pause Filter Feature.
> This feature creates a new field in the VMCB called Pause
> Filter Count. If Pause Filter Count is greater than 0 and
> ntercepting PAUSEs is enabled, the processor will increment
> an internal counter when a PAUSE instruction occurs instead
> of intercepting. When the internal counter reaches the
> Pause Filter Count value, a PAUSE intercept will occur.
>
> This feature can be used to detect contended spinlocks,
> especially when the lock holding VCPU is not scheduled.
> Rescheduling another VCPU prevents the VCPU seeking the
> lock from wasting its quantum by spinning idly.
>
> Experimental results show that most spinlocks are held
> for less than 1000 PAUSE cycles or more than a few
> thousand. Default the Pause Filter Counter to 3000 to
> detect the contended spinlocks.
>
> Processor support for this feature is indicated by a CPUID
> bit.
>
> On a 24 core system running 4 guests each with 16 VCPUs,
> this patch improved overall performance of each guest's
> 32 job kernbench by approximately 1%. Further performance
> improvement may be possible with a more sophisticated
> yield algorithm.
>
> -Mark Langsdorf
> Operating System Research Center
> AMD
>
> Signed-off-by: Mark Langsdorf <mark.langsdorf@....com>
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index ef43a18..14dab13 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -575,6 +576,12 @@ static void init_vmcb(struct vcpu_svm *svm)
>
> svm->nested_vmcb = 0;
> svm->vcpu.arch.hflags = HF_GIF_MASK;
> +
> + if (svm_has(SVM_FEATURE_PAUSE_FILTER)) {
> + control->pause_filter_count = 5000;
The commit message says something about 3000.
> + control->intercept |= (1ULL << INTERCEPT_PAUSE);
> + }
> +
> }
>
> static int svm_vcpu_reset(struct kvm_vcpu *vcpu)
--
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