Re: [PATCH v6 4/5] KVM: VMX: Allow direct access to MSR_IA32_SPEC_CTRL

[Jim Mattson]

On Thu, Feb 1, 2018 at 1:59 PM, KarimAllah Ahmed <karahmed@xxxxxxxxx> wrote:
> [ Based on a patch from Ashok Raj <ashok.raj@xxxxxxxxx> ]
>
> 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@xxxxxxxxx>
> Cc: Arjan Van De Ven <arjan.van.de.ven@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Jun Nakajima <jun.nakajima@xxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
> Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Ashok Raj <ashok.raj@xxxxxxxxx>
> Signed-off-by: KarimAllah Ahmed <karahmed@xxxxxxxxx>
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>

Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>