Re: [Xen-devel] about fully UMIP support in Xen

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.04.17 at 11:48, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> On 4/19/2017 5:18 PM, Jan Beulich wrote:
>>>>> On 19.04.17 at 10:48, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
>>>     I saw that commit 8c14e5f provides emulations for UMIP affected
>>> instructions. But realized that xen does not have logic to expose UMIP
>>> feature to guests - you have sent out one in
>>> https://lists.xenproject.org/archives/html/xen-devel/2016-12/msg00552.html 
>>> to emulate the cpuid leaf, but seems it was only a software solution and
>>> have not get merged yet.
>>>     So I wonder do you have any specific plan to fully support the UMIP,
>>> i.e. in Xen 4.10? :-)
>> It would be nice, but I think there are caveats: While PV guests
>> _shouldn't_ use any of the affected instructions, we can't blindly
>> assume they don't. Hence we'd have to emulate them (producing
>> to be determined data, e.g. all zeros). Luckily we don't have to
>> care about VM86 mode, as for code running there emulating the
>> instructions would be mandatory (as they're e.g. needed for CPU
>> family detection, since the recommended approach to tell [iirc]
>> 286 from 386 doesn't reliably work there).
> 
> Thanks, Jan.
> You mean if UMIP is enabled in xen, and dom0 triggers affected 
> instructions, 0 should
> be returned?

Yes.

> Does hypervisor need to differentiate dom0 kernel and its 
> user space?

If we want to para-virtualize the feature, then yes. Otherwise
we can't assume the guest kernel would deal with user mode faults,
so we'd have to. Arguably there could be a non-default mode in
which we don't (forcing such applications to get a signal or crash).

Jan


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