Re: [PATCH] x86/cpu-policy: Extend the guest max policy max leaf/subleaves

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 30/10/2024 8:59 am, Roger Pau Monné wrote:
> On Tue, Oct 29, 2024 at 05:55:05PM +0000, Andrew Cooper wrote:
>> diff --git a/xen/arch/x86/cpu-policy.c b/xen/arch/x86/cpu-policy.c
>> index b6d9fad56773..78bc9872b09a 100644
>> --- a/xen/arch/x86/cpu-policy.c
>> +++ b/xen/arch/x86/cpu-policy.c
>> @@ -391,6 +391,27 @@ static void __init calculate_host_policy(void)
>>      p->platform_info.cpuid_faulting = cpu_has_cpuid_faulting;
>>  }
>>  
>> +/*
>> + * Guest max policies can have any max leaf/subleaf within bounds.
>> + *
>> + * - Some incoming VMs have a larger-than-necessary feat max_subleaf.
>> + * - Some VMs we'd like to synthesise leaves not present on the host.
>> + */
>> +static void __init guest_common_max_leaves(struct cpu_policy *p)
>> +{
>> +    p->basic.max_leaf       = ARRAY_SIZE(p->basic.raw) - 1;
>> +    p->feat.max_subleaf     = ARRAY_SIZE(p->feat.raw) - 1;
>> +    p->extd.max_leaf        = 0x80000000U + ARRAY_SIZE(p->extd.raw) - 1;
>> +}
>> +
>> +/* Guest default policies inherit the host max leaf/subleaf settings. */
>> +static void __init guest_common_default_leaves(struct cpu_policy *p)
>> +{
>> +    p->basic.max_leaf       = host_cpu_policy.basic.max_leaf;
>> +    p->feat.max_subleaf     = host_cpu_policy.feat.max_subleaf;
>> +    p->extd.max_leaf        = host_cpu_policy.extd.max_leaf;
>> +}
> I think this what I'm going to ask is future work.  After the
> modifications done to the host policy by max functions
> (calculate_{hvm,pv}_max_policy()) won't the max {sub,}leaf adjustments
> better be done taking into account the contents of the policy, rather
> than capping to the host values?
>
> (note this comment is strictly for guest_common_default_leaves(), the
> max version is fine using ARRAY_SIZE).

I'm afraid I don't follow.

calculate_{pv,hvm}_max_policy() don't modify the host policy.

~Andrew