From mboxrd@z Thu Jan  1 00:00:00 1970
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: vtime accounting
Date: Tue, 14 Mar 2017 18:09:45 +0100
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Cc: Christoffer Dall <christoffer.dall@linaro.org>,
        kvm@vger.kernel.org, Marc Zyngier <marc.zyngier@arm.com>,
        Rik van Riel <riel@redhat.com>
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        Christoffer Dall <cdall@linaro.org>
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On 14/03/2017 17:58, Radim Krčmář wrote:
>> I assume there's a good reason why we call guest_enter() and
>> guest_exit() in the hot path on every KVM architecture?
> I consider myself biased when it comes to jiffies, so no judgement. :)
> 
> From what I see, the mode switch is used only for statistics.

vtime is only for statistics, but guest_enter/exit are important because
they enter an RCU extended quiescent state.  This means that (physical)
CPUs running a guest are effectively "off" from the point of view of the
RCU accounting machinery.  Not having to perform any RCU work is very
good for jitter.

Paolo