Re: [Xen-devel] [PATCH v2 10/30] xen/x86: Annotate VM applicability in featureset

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.02.16 at 15:53, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 15/02/16 14:50, Jan Beulich wrote:
>>>>> On 15.02.16 at 15:38, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> On 15/02/16 09:20, Jan Beulich wrote:
>>>>>>> On 12.02.16 at 18:42, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>>> On 12/02/16 17:05, Jan Beulich wrote:
>>>>>>>>> On 05.02.16 at 14:42, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>>>>>  #define X86_FEATURE_MWAITX        ( 3*32+29) /*   MWAIT extension 
>>>>> (MONITORX/MWAITX) */
>>>>>> Why not exposed to HVM (also for _MWAIT as I now notice)?
>>>>> Because that is a good chunk of extra work to support.  We would need to
>>>>> use 4K monitor widths, and extra p2m handling.
>>>> I don't understand: The base (_MWAIT) feature being exposed to
>>>> guests today, and kernels making use of the feature when available
>>>> suggests to me that things work. Are you saying you know
>>>> otherwise? (And if there really is a reason to mask the feature all of
>>>> the sudden, this should again be justified in the commit message.)
>>> PV guests had it clobbered by Xen in traps.c
>>>
>>> HVM guests have:
>>>
>>> vmx.c:
>>>     case EXIT_REASON_MWAIT_INSTRUCTION:
>>>     case EXIT_REASON_MONITOR_INSTRUCTION:
>>> [...]
>>>     hvm_inject_hw_exception(TRAP_invalid_op, HVM_DELIVER_NO_ERROR_CODE);
>>>         break;
>>>
>>> and svm.c:
>>>     case VMEXIT_MONITOR:
>>>     case VMEXIT_MWAIT:
>>>         hvm_inject_hw_exception(TRAP_invalid_op, HVM_DELIVER_NO_ERROR_CODE);
>>>         break;
>>>
>>> I don't see how a guest could actually use this feature.
>> Do you see the respective intercepts getting enabled anywhere?
>> (I don't outside of nested code, which I didn't check in detail.)
> 
> Yes - the intercepts are always enabled to prevent the guest actually
> putting the processor to sleep.

Hmm, you're right, somehow I've managed to ignore the relevant
lines grep reported. Yet - how do things work then, without the
MWAIT feature flag currently getting cleared?

Jan


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