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Re: [Xen-devel] [PATCH v2 8/9] x86/CPUID: drop INVPCID dependency on PCID
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 18 Sep 2019 12:59:29 +0100
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- Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 18 Sep 2019 11:59:36 +0000
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On 18/09/2019 10:10, Jan Beulich wrote:
> 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.
Oh. Of course. I'm not confused as to why that was present to being with.
Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
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