Re: [PATCH v2 5/9] x86/spec-ctrl: Record the last write to MSR_SPEC_CTRL

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.01.2022 14:29, Andrew Cooper wrote:
> In some cases, writes to MSR_SPEC_CTRL do not have interesting side effects,
> and we should implement lazy context switching like we do with other MSRs.
> 
> In the short term, this will be used by the SVM infrastructure, but I expect
> to extend it to other contexts in due course.
> 
> Introduce cpu_info.last_spec_ctrl for the purpose, and cache writes made from
> the boot/resume paths.  The value can't live in regular per-cpu data when it
> is eventually used for PV guests when XPTI might be active.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Technically
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
But ...

> --- a/xen/arch/x86/acpi/power.c
> +++ b/xen/arch/x86/acpi/power.c
> @@ -296,7 +296,10 @@ static int enter_state(u32 state)
>      ci->spec_ctrl_flags |= (default_spec_ctrl_flags & SCF_ist_wrmsr);
>  
>      if ( boot_cpu_has(X86_FEATURE_IBRSB) )
> +    {
>          wrmsrl(MSR_SPEC_CTRL, default_xen_mcu_opt_ctrl);
> +        ci->last_spec_ctrl = default_xen_mcu_opt_ctrl;
> +    }

... wouldn't we better have a write_spec_ctrl() helper doing both,
perhaps with the check-if-same logic added as well (overridable by
a "force" boolean input, unless the cases where the write cannot be
skipped can be entirely determined from previous and new values)?

Jan