linux-kernel - Re: [PATCH v6 4/5] KVM: VMX: Allow direct access to MSR_IA32_SPEC

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Date:   Fri, 2 Feb 2018 09:57:34 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     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>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ashok Raj <ashok.raj@...el.com>
Subject: Re: [PATCH v6 4/5] KVM: VMX: Allow direct access to MSR_IA32_SPEC_CTRL

On Thu, Feb 1, 2018 at 1:59 PM, KarimAllah Ahmed <karahmed@...zon.de> wrote:
> [ Based on a patch from Ashok Raj <ashok.raj@...el.com> ]
>
> Add direct access to MSR_IA32_SPEC_CTRL for guests. This is needed for
> guests that will only mitigate Spectre V2 through IBRS+IBPB and will not
> be using a retpoline+IBPB based approach.
>
> To avoid the overhead of saving and restoring the MSR_IA32_SPEC_CTRL for
> guests that do not actually use the MSR, only start saving and restoring
> when a non-zero is written to it.
>
> No attempt is made to handle STIBP here, intentionally. Filtering STIBP
> may be added in a future patch, which may require trapping all writes
> if we don't want to pass it through directly to the guest.
>
> [dwmw2: Clean up CPUID bits, save/restore manually, handle reset]
>
> Cc: Asit Mallick <asit.k.mallick@...el.com>
> Cc: Arjan Van De Ven <arjan.van.de.ven@...el.com>
> Cc: Dave Hansen <dave.hansen@...el.com>
> Cc: Andi Kleen <ak@...ux.intel.com>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Tim Chen <tim.c.chen@...ux.intel.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Jun Nakajima <jun.nakajima@...el.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: David Woodhouse <dwmw@...zon.co.uk>
> Cc: Greg KH <gregkh@...uxfoundation.org>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Ashok Raj <ashok.raj@...el.com>
> Signed-off-by: KarimAllah Ahmed <karahmed@...zon.de>
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>

Reviewed-by: Jim Mattson <jmattson@...gle.com>