Re: [PATCH v4 1/6] x86: suppress ERMS for internal use when MISC_ENABLE.FAST_STRING is clear

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.01.2025 11:11, Jan Beulich wrote:
> --- a/xen/arch/x86/cpu-policy.c
> +++ b/xen/arch/x86/cpu-policy.c
> @@ -487,6 +487,12 @@ static void __init guest_common_max_feat
>       */
>      if ( test_bit(X86_FEATURE_RTM, fs) )
>          __set_bit(X86_FEATURE_RTM_ALWAYS_ABORT, fs);
> +
> +    /*
> +     * We expose MISC_ENABLE to guests, so our internal clearing of ERMS when
> +     * FAST_STRING is not set should not affect the view of migrating-in 
> guests.
> +     */
> +    __set_bit(X86_FEATURE_ERMS, fs);
>  }
>  
>  static void __init guest_common_default_feature_adjustments(uint32_t *fs)
> @@ -591,6 +597,15 @@ static void __init guest_common_feature_
>       */
>      if ( host_cpu_policy.feat.ibrsb )
>          __set_bit(X86_FEATURE_IBPB, fs);
> +
> +    /*
> +     * We expose MISC_ENABLE to guests, so our internal clearing of ERMS when
> +     * FAST_STRING is not set should not propagate to guest view.  Guests can
> +     * judge on their own whether to ignore the CPUID bit when the MSR bit is
> +     * clear.
> +     */
> +    if ( raw_cpu_policy.feat.erms )
> +        __set_bit(X86_FEATURE_ERMS, fs);
>  }
>  
>  static void __init calculate_pv_max_policy(void)

While doing unrelated work in this area I noticed that this hunk is wrong,
and wants to be

@@ -567,6 +573,16 @@ static void __init guest_common_default_
         __clear_bit(X86_FEATURE_RTM, fs);
         __set_bit(X86_FEATURE_RTM_ALWAYS_ABORT, fs);
     }
+
+    /*
+     * We expose MISC_ENABLE to guests, so our internal clearing of ERMS when
+     * FAST_STRING is not set should not propagate to guest view.  Guests can
+     * judge on their own whether to ignore the CPUID bit when the MSR bit is
+     * clear.  The bit being uniformly set in the max policies, we only need
+     * to clear it here (if hardware doesn't have it).
+     */
+    if ( !raw_cpu_policy.feat.erms )
+        __clear_bit(X86_FEATURE_ERMS, fs);
 }
 
 static void __init guest_common_feature_adjustments(uint32_t *fs)

instead. Fixed locally.

Jan