linux-kernel - Re: [PATCH v4 2/5] KVM: x86: Add IBPB support

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Message-ID: <4c3f3acf-b07b-cb6a-8ca1-e850980eda0d@amazon.com>
Date:   Wed, 31 Jan 2018 18:17:04 +0100
From:   KarimAllah Ahmed <karahmed@...zon.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        KarimAllah Ahmed <karahmed@...zon.de>
CC:     the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Ashok Raj <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        "Tim Chen" <tim.c.chen@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Andi Kleen" <ak@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Dan Williams" <dan.j.williams@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH v4 2/5] KVM: x86: Add IBPB support

On 01/31/2018 05:55 PM, Paolo Bonzini wrote:
> On 31/01/2018 11:50, Jim Mattson wrote:
>>> +       if (to_vmx(vcpu)->save_spec_ctrl_on_exit) {
>>> +               nested_vmx_disable_intercept_for_msr(
>>> +                               msr_bitmap_l1, msr_bitmap_l0,
>>> +                               MSR_IA32_PRED_CMD,
>>> +                               MSR_TYPE_R);
>>> +       }
>> I don't think this should be predicated on
>> "to_vmx(vcpu)->save_spec_ctrl_on_exit." Why not just
>> "guest_cpuid_has(vcpu, X86_FEATURE_IBPB)"? Also, the final argument to
>> nested_vmx_disable_intercept_for_msr should be MSR_TYPE_W rather than
>> MSR_TYPE_R.
> 
> In fact this MSR can even be passed down unconditionally, since it needs
> no save/restore and has no ill performance effect on the sibling
> hyperthread.
> 
> Only MSR_IA32_SPEC_CTRL needs to be conditional on
> "to_vmx(vcpu)->save_spec_ctrl_on_exit".

That used to be the case in an earlier version. There seems to be two
opinions here:

1) Pass it only if CPUID for the guest has it.
2) Pass it unconditionally.

I do not really have a preference.


> 
> Paolo
> 
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