Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to MSR_IA32_SPEC_CTRL

[Raj, Ashok]

Hi Karim

On Wed, Jan 31, 2018 at 08:37:46PM +0100, KarimAllah Ahmed 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.

With these changes SPEC_CTRL is properly exposed to the guest when using
latest Qemu.

> 
> To avoid the overhead of atomically saving and restoring the
> MSR_IA32_SPEC_CTRL for guests that do not actually use the MSR, only
> add_atomic_switch_msr 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]
>