Re: [PATCH] x86/cpu-policy: set up host policy earlier

[Marek Marczykowski <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Tue, Apr 07, 2026 at 08:46:39AM +0200, Jan Beulich wrote:
> In order to use cpu_has_* expanding to host_cpu_policy.*, the host policy
> needs setting up alongside boot_cpu_data.x86_capability[]. Arrange for
> that towards the end of identify_cpu().
> 
> Fixes: 894bd7617924 ("x86/Intel: use host CPU policy for ARAT checking")
> Reported-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Tested-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>

> ---
> In the longer run we may need to (also) update the host policy from
> setup_{force,clear}_cpu_cap().
> 
> This really was part of one of the XSTATE cleanup patches at the head of
> the AMX series. This explains the specific placement ahead of the call to
> xstate_init()). Likely anything from there downwards shouldn't really
> live in identify_cpu() anyway.
> 
> --- a/xen/arch/x86/cpu/common.c
> +++ b/xen/arch/x86/cpu/common.c
> @@ -586,6 +586,8 @@ void identify_cpu(struct cpuinfo_x86 *c)
>       }
>  
>       /* Now the feature flags better reflect actual CPU features! */
> +     if (c == &boot_cpu_data)
> +             calculate_host_policy();
>  
>       xstate_init(c);
>  
> --- a/xen/arch/x86/cpu-policy.c
> +++ b/xen/arch/x86/cpu-policy.c
> @@ -359,7 +359,7 @@ void calculate_raw_cpu_policy(void)
>      /* Was already added by probe_cpuid_faulting() */
>  }
>  
> -static void __init calculate_host_policy(void)
> +void __init calculate_host_policy(void)
>  {
>      struct cpu_policy *p = &host_cpu_policy;
>      unsigned int max_extd_leaf;
> @@ -904,6 +904,7 @@ static void __init calculate_hvm_def_pol
>  
>  void __init init_guest_cpu_policies(void)
>  {
> +    /* Do this a 2nd time to account for setup_{clear,force}_cpu_cap() uses. 
> */
>      calculate_host_policy();
>  
>      if ( IS_ENABLED(CONFIG_PV) )
> --- a/xen/arch/x86/include/asm/cpu-policy.h
> +++ b/xen/arch/x86/include/asm/cpu-policy.h
> @@ -30,4 +30,10 @@ void recalculate_cpuid_policy(struct dom
>   */
>  void calculate_raw_cpu_policy(void);
>  
> +/*
> + * Collect the host CPU policy.  Called after collecting enough CPUID output,
> + * and again after all feature overrides have been put in place.
> + */
> +void calculate_host_policy(void);
> +
>  #endif /* X86_CPU_POLICY_H */

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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