linux-kernel - Re: [PATCH][KVM][retry 1] Add support for Pause Filtering to AMD SVM

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Message-ID: <20090511141503.GC6175@elte.hu>
Date:	Mon, 11 May 2009 16:15:03 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Mark Langsdorf <mark.langsdorf@....com>,
	Joerg Roedel <joerg.roedel@....com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH][KVM][retry 1] Add support for Pause Filtering to AMD
	SVM


* Avi Kivity <avi@...hat.com> wrote:

>>  +static int pause_interception(struct vcpu_svm *svm, struct kvm_run 
>> *kvm_run)
>> +{
>> +	/* Simple yield */
>> +	vcpu_put(&svm->vcpu);
>> +	schedule();
>> +	vcpu_load(&svm->vcpu);
>> +	return 1;
>> +
>
> Ingo, will this do anything under CFS, or will CFS note that 
> nothing has changed in the accounting and reschedule us 
> immediately?

The scheduler will yield to another task only if the current task 
has become ineligible. I.e schedule() is largely a NOP on 
TASK_RUNNING tasks (i.e. here).

I.e. this is a somewhat poor solution as far as scheduling goes. But 
i'm wondering what the CPU side does. Can REP-NOP really take 
thousands of cycles? If yes, under what circumstances?

	Ingo
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