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 20:00:24 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Jim Mattson <jmattson@...gle.com>,
        KarimAllah Ahmed <karahmed@...zon.de>
Cc:     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 11:53 -0800, Jim Mattson wrote:
> On Wed, Jan 31, 2018 at 11:37 AM, KarimAllah Ahmed  wrote:
> 
> > +
> > +       if (to_vmx(vcpu)->save_spec_ctrl_on_exit) {
> > +               nested_vmx_disable_intercept_for_msr(
> > +                               msr_bitmap_l1, msr_bitmap_l0,
> > +                               MSR_IA32_SPEC_CTRL,
> > +                               MSR_TYPE_R | MSR_TYPE_W);
> > +       }
> > +
> 
> As this is written, L2 will never get direct access to this MSR until
> after L1 writes it.  What if L1 never writes it? The condition should
> really be something that captures, "if L0 is willing to yield this MSR
> to the guest..."

I'm still kind of lost here, but don't forget the requirement that the
MSR must *not* be passed through for direct access by L1 or L2 guests,
unless that ->save_spec_ctrl_on_exit flag is set.

Because that's what makes us set it back to zero on vmexit.

So the above condition doesn't look *so* wrong to me. Perhaps the issue
is that we're missing a way for L2 to actually cause that flag to get
set?
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