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Re: [PATCH] x86/cpu-policy: Fix x2APIC visibility for PV guests
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 29 Feb 2024 19:09:31 +0000
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 29 Feb 2024 19:09:37 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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
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