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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.07.2026 17:21, Andrew Cooper wrote:
> On 07/04/2026 7:46 am, 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>
>> ---
>> In the longer run we may need to (also) update the host policy from
>> setup_{force,clear}_cpu_cap().

I'm noticing only while making the change that I did already suggest here
what ...

>> --- 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();
> 
> I'm sorry, but no.  The cleanup work to CPUID handling is in an attempt
> to purge this antipattern.
> 
> Double-initialising data structures *is* the entire reason we have
> order-of-boot problems in the first place.  Having a structure which is
> initialised but wrong is itself wrong.
> 
> Really, I should insist on a revert of switching cpu_has_* to use
> host_policy.  It was not adequately tested and is not compatible with
> how Xen starts up.
> 
> But as I expect you're going to object to that feedback, I'll tolerate
> setup_*_cpu_cap() keeping the host policy correct as they're called. 
> The easy option is to call x86_cpu_featureset_to_policy() each time. 

... you're effectively (see my earlier reply) asking for here.

Jan