linux-kernel - Re: [PATCH v7 10/12] KVM/x86/lbr: lazy save the guest lbr stack

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Message-ID: <20190708151141.GL31027@tassilo.jf.intel.com>
Date:   Mon, 8 Jul 2019 08:11:41 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Wei Wang <wei.w.wang@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, pbonzini@...hat.com, kan.liang@...el.com,
        mingo@...hat.com, rkrcmar@...hat.com, like.xu@...el.com,
        jannh@...gle.com, arei.gonglei@...wei.com, jmattson@...gle.com
Subject: Re: [PATCH v7 10/12] KVM/x86/lbr: lazy save the guest lbr stack

> I don't understand a word of that.
> 
> Who cares if the LBR MSRs are touched; the guest expects up-to-date
> values when it does reads them.

This is for only when the LBRs are disabled in the guest.

It doesn't make sense to constantly save/restore disabled LBRs, which
would be a large overhead for guests that don't use LBRs. 

When the LBRs are enabled they always need to be saved/restored as you
say.

-Andi