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

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Date:   Thu, 01 Feb 2018 13:25:37 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        KarimAllah Ahmed <karahmed@...zon.com>
Cc:     Jim Mattson <jmattson@...gle.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 23:26 -0500, Konrad Rzeszutek Wilk wrote:
> 
> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > index 6a9f4ec..bfc80ff 100644
> > --- a/arch/x86/kvm/vmx.c
> > +++ b/arch/x86/kvm/vmx.c
> > @@ -594,6 +594,14 @@ struct vcpu_vmx {
> >  #endif
> >  
> >       u64                   arch_capabilities;
> > +     u64                   spec_ctrl;
> > +
> > +     /*
> > +      * This indicates that:
> > +      * 1) guest_cpuid_has(X86_FEATURE_IBRS) == true &&
> > +      * 2) The guest has actually initiated a write against the MSR.
> > +      */
> > +     bool spec_ctrl_used;
> >  
> >       /*
> >        * This indicates that:

Thanks for persisting with the details here, Karim. In addition to
Konrad's heckling at the comments, I'll add my own request to his...

I'd like the comment for spec_ctrl_used to explain why it isn't
entirely redundant with the spec_ctrl_intercepted() function.

Without nesting, I believe it *would* be redundant, but the difference
comes when an L2 is running for which L1 has not permitted the MSR to
be passed through. That's when we have spec_ctrl_used = true but the
MSR *isn't* actually passed through in the active msr_bitmap.

Question: if spec_ctrl_used is always equivalent to the intercept bit
in the vmcs01.msr_bitmap, just not the guest bitmap... should we ditch
it and always use the bit from the vmcs01.msr_bitmap?

Sorry :)
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