Re: [PATCH] x86/cpu-policy: Fix x2APIC visibility for PV guests

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

On 29/02/2024 12:47 pm, Jan Beulich wrote:
>> @@ -830,11 +846,10 @@ void recalculate_cpuid_policy(struct domain *d)
>>      }
>>  
>>      /*
>> -     * Allow the toolstack to set HTT, X2APIC and CMP_LEGACY.  These bits
>> +     * Allow the toolstack to set HTT and CMP_LEGACY.  These bits
>>       * affect how to interpret topology information in other cpuid leaves.
>>       */
>>      __set_bit(X86_FEATURE_HTT, max_fs);
>> -    __set_bit(X86_FEATURE_X2APIC, max_fs);
>>      __set_bit(X86_FEATURE_CMP_LEGACY, max_fs);
>>  
>>      /*
> ... these adjustments, just still in calculate_pv_featureset(). I
> haven't gone further backwards to check if/when this exposure has
> really appeared. I wouldn't be surprised if it's been like that
> for all the time since we gained x2APIC support in the hypervisor.

4.7 has:

cpufeatureset.h:140:XEN_CPUFEATURE(X2APIC,        1*32+21) /*!A Extended
xAPIC */

so will expose it to PV guests.

4.6 has this gem I'd forgotten, in pv_cpuid():

        if ( !cpu_has_apic )
           __clear_bit(X86_FEATURE_X2APIC % 32, &c);

but I don't see any (sensible) logic to hide x2APIC from the view of a
PV guest.

So PV guests really have see x2APIC from the host forever (and
irrespective of Xen's x2APIC support).

~Andrew