Re: [Xen-devel] [PATCH v2 8/9] x86/CPUID: drop INVPCID dependency on PCID

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.09.2019 21:59, Andrew Cooper wrote:
> On 17/09/2019 07:17, Jan Beulich wrote:
>> PCID validly depends on LM, as it can be enabled in Long Mode only.
>> INVPCID, otoh, can be used not only without PCID enabled, but also
>> outside of Long Mode altogether. In both cases its functionality is
>> simply restricted to PCID 0, which is sort of expected as no other PCID
>> can be activated there.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>
>> --- a/xen/tools/gen-cpuid.py
>> +++ b/xen/tools/gen-cpuid.py
>> @@ -218,10 +218,6 @@ def crunch_numbers(state):
>>          #
>>          # SSE4_2: [POPCNT]
>>  
>> -        # The INVPCID instruction depends on PCID infrastructure being
>> -        # available.
>> -        PCID: [INVPCID],
> 
> I take it you haven't tried running a shadow guest with this change in
> place.
> 
> For !EPT, we clear ENABLE_INVPCID so the instruction will #UD within the
> guest.
> 
> We could in principle allow the use of INVPCID with shadow guests, but
> that would involve teaching Xen how to cope with VMEXIT_REASON_INVPCID
> (which is predicated on INVLPG exiting) and wire the flush back into the
> shadow logic.
> 
> Perhaps the comment could be a little clearer, but it is no accident
> that dependency is in place.

I'm afraid I don't follow: Both features are marked H in the
public header, so I don't see how shadow mode considerations
apply here at all.

Jan

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel