Re: [PATCH 6/8] x86/spec-ctrl: Use common MSR_SPEC_CTRL logic for AMD

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.01.2022 09:44, Andrew Cooper wrote:
> Currently, amd_init_ssbd() works by being the only write to MSR_SPEC_CTRL in
> the system.  This ceases to be true when using the common logic.
> 
> Include AMD MSR_SPEC_CTRL in has_spec_ctrl to activate the common paths, and
> introduce an AMD specific block to control alternatives.
> 
> For now, only configure alternatives for HVM.  PV will require more work.
> 
> This is a reasonably large change for low level defaults in the common case,
> but should have no practical change in behaviour.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

One nit:

> --- a/xen/arch/x86/cpu/amd.c
> @@ -1031,12 +1033,32 @@ void __init init_speculation_mitigations(void)
>          }
>      }
>  
> +    /* AMD hardware: MSR_SPEC_CTRL alternatives setup. */
> +    if ( boot_cpu_has(X86_FEATURE_IBRS) )
> +    {
> +        /*
> +         * Virtualising MSR_SPEC_CTRL for guests depends on SVM support, 
> which
> +         * on real hardware matches the availability of MSR_SPEC_CTRL in the
> +         * first place.
> +         *
> +         * No need for SCF_ist_wrmsr because, because Xen's value is restored
> +         * atomically WRT NMIs in the VMExit path.

There's one "because" too many here.

Jan