linux-kernel - Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to MSR_IA32_SPEC

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Date:   Wed, 31 Jan 2018 22:05:29 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     KarimAllah Ahmed <karahmed@...zon.com>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ashok Raj <ashok.raj@...el.com>
Subject: Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to
 MSR_IA32_SPEC_CTRL



On Wed, 2018-01-31 at 13:18 -0800, Jim Mattson wrote:
> On Wed, Jan 31, 2018 at 12:21 PM, David Woodhouse  wrote:
> 
> > 
> > Reading and writing this MSR is expensive. And if it's yielded to the
> > guest in the MSR bitmap, that means we have to save its value on vmexit
> > and set it back to zero.
>
> Agreed. But my point is that if it's not yielded to the guest in the
> MSR bitmap, then we don't have to save its value on VM-exit and set it
> back to zero. The vmcs02 MSR bitmap is reconstructed on every L1->L2
> transition. Sometimes, it will yield the MSR and sometimes it won't.

Strictly: if SPEC_CTRL is not already set to 1 *and* hasn't been
yielded to the guest in the MSR bitmap, then we don't have to set it
back to zero.

If L1 decides it's *always* going to trap and never pass through, but
the value is already set to non-zero, we need to get that case right.

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