Re: [Xen-devel] [PATCH for-4.11] x86: Use spec_ctrl_{enter, exit}_idle() in the S3/S5 path

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.04.18 at 12:43, <andrew.cooper3@xxxxxxxxxx> wrote:
> This avoids opencoding the functionality (and missing one bit of it), and and
> some comments explaining what is going on.

Missing which bit of it? The MSR writes aren't strictly necessary afaict, and
functionally clearing bti_ist_info is all that's needed for the entry path of
interest, while clearing use_shadow_spec_ctrl is all that's needed for the
exit-to-Xen path. Hence I'm don't see (yet) what bug it is you think this
fixes.

Also s/and$/add/.

> --- a/xen/arch/x86/acpi/power.c
> +++ b/xen/arch/x86/acpi/power.c
> @@ -213,7 +213,8 @@ static int enter_state(u32 state)
>          error = 0;
>  
>      ci = get_cpu_info();
> -    ci->use_shadow_spec_ctrl = 0;
> +    spec_ctrl_enter_idle(ci);
> +    /* Avoid NMI/#MC using MSR_SPEC_CTRL until we've reloaded microcode. */
>      ci->bti_ist_info = 0;
>  
>      ACPI_FLUSH_CPU_CACHE();
> @@ -257,10 +258,9 @@ static int enter_state(u32 state)
>      if ( !recheck_cpu_features(0) )
>          panic("Missing previously available feature(s).");
>  
> +    /* Re-enabled default NMI/#MC use of MSR_SPEC_CTRL. */
>      ci->bti_ist_info = default_bti_ist_info;
> -    asm volatile (ALTERNATIVE("", "wrmsr", X86_FEATURE_XEN_IBRS_SET)
> -                  :: "a" (SPEC_CTRL_IBRS), "c" (MSR_SPEC_CTRL), "d" (0)
> -                  : "memory");
> +    spec_ctrl_exit_idle(ci);

The use of "idle" methods is a slight abuse here (fundamentally "idle" is
about C states), but yes, perhaps in a way a system in S3/S5 can be
viewed sort of (in a heavyweight way) idle.

Jan



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