linux-kernel - Re: [PATCH 9/9] KVM: VMX: automatic PLE window maximum

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Date:	Wed, 20 Aug 2014 14:41:12 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Gleb Natapov <gleb@...nel.org>,
	Raghavendra KT <raghavendra.kt@...ux.vnet.ibm.com>,
	Vinod Chegu <chegu_vinod@...com>, Hui-Zhi <hui-zhi.zhao@...com>
Subject: Re: [PATCH 9/9] KVM: VMX: automatic PLE window maximum

2014-08-20 09:16+0200, Paolo Bonzini:
> Il 19/08/2014 22:35, Radim Krčmář ha scritto:
> > Every increase of ple_window_grow creates potential overflows.
> > They are not serious, because we clamp ple_window and userspace is
> > expected to fix ple_window_max within a second.
> > ---
> I think avoiding overflows is better.  In fact, I think you should call
> this function for ple_window_max too.

(Ack, I just wanted to avoid the worst userspace error, which is why
 PW_max hasn't changed when PW_grow got smaller and we could overflow.)

> You could keep the ple_window_max variable to the user-set value.
> Whenever ple_window_grow or ple_window_max are changed, you can set an
> internal variable (let's call it ple_window_actual_max, but I'm not wed
> to this name) to the computed value, and then do:
> 
> 	if (ple_window_grow < 1 || ple_window_actual_max < ple_window)
> 		new = ple_window;
> 	else if (ple_window_grow < ple_window)
> 		new = max(ple_window_actual_max, old) * ple_window_grow;
> 	else
> 		new = max(ple_window_actual_max, old) + ple_window_grow;

Oh, I like that this can get rid of all overflows, ple_window_actual_max
(PW_effective_max?) is going to be set to
"ple_window_max [/-] ple_window_grow" in v2.

> (I think the || in the first "if" can be eliminated with some creativity
> in clamp_ple_window_max).

To do it, we'll want to intercept changes to ple_window as well.
(I disliked this patch a lot even before :)
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