Re: about fully UMIP support in Xen

[Yu Zhang <yu.c.zhang@linux.intel.com>]

On 4/19/2017 7:19 PM, Jan Beulich wrote:
>>>> On 19.04.17 at 11:48, <yu.c.zhang@linux.intel.com> wrote:
>> On 4/19/2017 5:18 PM, Jan Beulich wrote:
>>>>>> On 19.04.17 at 10:48, <yu.c.zhang@linux.intel.com> 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).

Thanks, Jan.
For UMIP is to be para-virtualized,  is it OK to give dom0 kernel the 
physical value
if instructions are triggered in the kernel?

And if the instructions are triggered in dom0 user space, the spec 
requires a #GP
fault, and we can return 0 to the application in the #GP fault handler, 
is it OK?

Yu


> Jan
>
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> https://lists.xen.org/xen-devel


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel