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    SubjectRe: [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@amazon.de> wrote:
    > [ Based on a patch from Ashok Raj <ashok.raj@intel.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@intel.com>
    > Cc: Arjan Van De Ven <arjan.van.de.ven@intel.com>
    > Cc: Dave Hansen <dave.hansen@intel.com>
    > Cc: Andi Kleen <ak@linux.intel.com>
    > Cc: Andrea Arcangeli <aarcange@redhat.com>
    > Cc: Linus Torvalds <torvalds@linux-foundation.org>
    > Cc: Tim Chen <tim.c.chen@linux.intel.com>
    > Cc: Thomas Gleixner <tglx@linutronix.de>
    > Cc: Dan Williams <dan.j.williams@intel.com>
    > Cc: Jun Nakajima <jun.nakajima@intel.com>
    > Cc: Paolo Bonzini <pbonzini@redhat.com>
    > Cc: David Woodhouse <dwmw@amazon.co.uk>
    > Cc: Greg KH <gregkh@linuxfoundation.org>
    > Cc: Andy Lutomirski <luto@kernel.org>
    > Cc: Ashok Raj <ashok.raj@intel.com>
    > Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
    > Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>

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

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