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

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Message-ID: <1517433465.18619.209.camel@amazon.co.uk>
Date:   Wed, 31 Jan 2018 21:17:45 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     Jim Mattson <jmattson@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.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>,
        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 13:05 -0800, Jim Mattson wrote:
> On Wed, Jan 31, 2018 at 1:00 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> 
> > Yes, but how would moving the field into struct loaded_vmcs do anything?
> >  Only vmon/vmoff would change anything in vmx->nested.vmcs02.
> 
> My suggestion was that nested_vmx_merge_msr_bitmap should set the
> vmcs02 version of save_spec_ctrl_on_exit based on the calculated value
> of the write permission bit for IA32_SPEC_CTRL in the vmcs02 MSR
> permission bitmap.
> 
> > Even then, L1 vmexits will also be penalized because L1 has probably
> > done an RDMSR/WRMSR on L2->L1 vmexit.  So I don't think it's an issue?
> 
> Yes, it sucks to be L1 in this situation.

Well... we *could* clear the save_spec_ctrl_on_exit flag and intercept
the MSR again, any time that the actual value of spec_ctrl is zero. 

I don't think we'd want to do that too aggressively, but there might be
something we could do there.
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