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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 30/10/2024 6:51 am, Jan Beulich wrote:
> On 29.10.2024 18:55, Andrew Cooper wrote:
>> We already have one migration case opencoded (feat.max_subleaf).  A more
>> recent discovery is that we advertise x2APIC to guests without ensuring that
>> we provide max_leaf >= 0xb.
>>
>> In general, any leaf known to Xen can be safely configured by the toolstack if
>> it doesn't violate other constraints.
>>
>> Therefore, introduce guest_common_{max,default}_leaves() to generalise the
>> special case we currently have for feat.max_subleaf, in preparation to be able
>> to provide x2APIC topology in leaf 0xb even on older hardware.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> Reviewed-by: Jan Beulich <jbeulich@suse.com>
>
> I'll have to update the AMX logic accordingly (maybe also the AVX10 one).

Yeah - I need to get back to your shrinking series too.

> I'd like to point out that this highlights a naming anomaly in
> x86_cpu_policies_are_compatible(): update_domain_cpu_policy() passes in
> the respective max policy as first argument. Imo the first parameter of
> the function would better be named "max" there.

That's covered in the documentation.  It made sense when I first planned
things, but that was many many iterations ago.

>
>> --- 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;
>> +}
> Which sadly still leaves open how to suitably shrink the max values,
> when they're larger than necessary (for the guest).

Only the toolstack can do the shrinking, and only as the about the final
step after optional features have been activated.

~Andrew