linux-kernel - Re: [KVM TSC trapping / migration 1/2] Add TSC trapping for SVM and VMX

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Date:	Thu, 6 Jan 2011 11:41:37 +0100
From:	Alexander Graf <agraf@...e.de>
To:	Zachary Amsden <zamsden@...hat.com>
Cc:	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	Zachary Amsden <zamsden@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [KVM TSC trapping / migration 1/2] Add TSC trapping for SVM and VMX


Am 06.01.2011 um 11:10 schrieb Zachary Amsden <zamsden@...hat.com>:

> Reasons to trap the TSC are numerous, but we want to avoid it as much
> as possible for performance reasons.
> 
> We provide two conservative modes via modules parameters and userspace
> hinting.  First, the module can be loaded with "tsc_auto=1" as a module
> parameter, which turns on conservative TSC trapping only when it is
> required (when unstable TSC or faster KHZ CPU is detected).
> 
> For userspace hinting, we enable trapping only if necessary.  Userspace
> can hint that a VM needs a fixed frequency TSC, and also that SMP
> stability will be required.  In that case, we conservatively turn on
> trapping when it is needed.  In addition, users may now specify the
> desired TSC rate at which to run.  If this rate differs significantly
> from the host rate, trapping will be enabled.
> 
> There is also an override control to allow TSC trapping to be turned on
> or off unconditionally for testing.
> 
> We indicate to pvclock users that the TSC is being trapped, to allow
> avoiding overhead and directly using RDTSCP (only for SVM).  This
> optimization is not yet implemented.

When migrating, the implementation could switch from non-trapped to trapped, making it less attractive. The guest however does not get notified about this change. Same for the other way around.

Would it make sense to add a kvmclock interrupt to notify the guest of such a change?


Alex

> 
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