linux-kernel - Re: [PATCH RFC 2/2] KVM: x86/vPMU: ignore access to LBR-related MSRs

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Date:   Wed, 6 Dec 2017 09:55:43 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Jan Dakinevich <jan.dakinevich@...tuozzo.com>
Cc:     linux-kernel@...r.kernel.org,
        "Denis V . Lunev" <den@...tuozzo.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Kan Liang <kan.liang@...el.com>,
        Stephane Eranian <eranian@...gle.com>,
        Colin King <colin.king@...onical.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Jin Yao <yao.jin@...ux.intel.com>, kvm@...r.kernel.org
Subject: Re: [PATCH RFC 2/2] KVM: x86/vPMU: ignore access to LBR-related MSRs

On Wed, Dec 06, 2017 at 08:02:07PM +0300, Jan Dakinevich wrote:
> On Wed, 6 Dec 2017 07:57:28 -0800
> Andi Kleen <ak@...ux.intel.com> wrote:
> 
> > 
> > If you do all this it's only a small step to fully enable LBRs for
> > guests. 
> 
> It is quite simple in a case where guest LBR-related MSRs matches host
> ones. They could be handled by MSR load/store areas, I suppose.

There is already a LBR control to enable/disable I believe.
You don't want to save/restore all MSRs on every entry/exit 
because that would be slow. The normal Linux context switch can do it.
 
> 
> In other cases, it could be expected the different amount of these MSRs
> and different theirs base values (e.g. Nehalem vs Core). Guest MSRs
> could be both subset and superset of host MSRs, so additional efforts
> to support this would be required.

In this case ignoring would be sufficient I suppose. But for the case
when everything matches it should work.

-Andi