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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 15/02/16 15:02, Jan Beulich wrote:
>>>> On 15.02.16 at 15:53, <andrew.cooper3@citrix.com> wrote:
>> On 15/02/16 14:50, Jan Beulich wrote:
>>>>>> On 15.02.16 at 15:38, <andrew.cooper3@citrix.com> wrote:
>>>> On 15/02/16 09:20, Jan Beulich wrote:
>>>>>>>> On 12.02.16 at 18:42, <andrew.cooper3@citrix.com> wrote:
>>>>>> On 12/02/16 17:05, Jan Beulich wrote:
>>>>>>>>>> On 05.02.16 at 14:42, <andrew.cooper3@citrix.com> 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?

I have never observed it being used.  Do you have some local patches in
the SLES hypervisor?

There is some gross layer violation in xen/enlighten.c to pretend that
MWAIT is present to trick the ACPI code into evaluating _CST() methods
to report back to Xen.  (This is yet another PV-ism which will cause a
headache for a DMLite dom0)

~Andrew