Re: [Xen-devel] [PATCH v3 5/7] xen/x86: disable global pages for domains with XPTI active

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.03.18 at 10:44, <jgross@xxxxxxxx> wrote:
> On 27/03/18 10:33, Jan Beulich wrote:
>>>>> On 27.03.18 at 09:37, <jgross@xxxxxxxx> wrote:
>>> On 27/03/18 09:23, Jan Beulich wrote:
>>>>>>> On 27.03.18 at 09:14, <jgross@xxxxxxxx> wrote:
>>>>> I just realized that using read_cr4() | X86_CR4_PGE is wrong. We would
>>>>> keep bits switched on which a pv domain is allowed to modify (plus
>>>>> CR4_TSD eventually).
>>>>>
>>>>> Do we really want that?
>>>>
>>>> Does it matter what exact CR4 settings we run with when it's not
>>>> a PV guest that's in context, and when we don't depend on the
>>>> settings ourselves? I don't think it does, and HVM guests run with
>>>> their own CR4 anyway. In fact there may end up being cases
>>>> where we won't need to switch CR4 another time when we come
>>>> here the next time with v being a PV vCPU.
>>>
>>> I could imagine that there is some performance impact. cr4.tsd set
>>> might make rdtsc a little bit slower as an additional privilege level
>>> check is needed.
>> 
>> Quite possible, indeed. Another opinion on the route to take
>> would be helpful here. Andrew?
> 
> I could mask away tsd, of course. I need to do so for pcide already,
> so that would be just another bit reset in the mask.

Fine with me.

Jan


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