Re: about fully UMIP support in Xen

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

On 4/19/2017 5:59 PM, Andrew Cooper wrote:
> On 19/04/17 10:48, Yu Zhang 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? Does hypervisor need to differentiate dom0 kernel and its
>> user space?
>>
>>> For HVM guests the feature is of no interest to Xen itself anyway,
>>> i.e. all we'd need is allow them to enable the CR4 bit (which my
>>> emulation patch did as a side effect, but that patch has been
>>> deferred until after Andrew manages to put in some more CPUID
>>> work; that single hunk could certainly be split out if desired).
>> By "CPUID work", I guess you changes needed in the
>> xen/tools/gen-cpuid.py,
>> not just emulating one to the guest, right?
> There are two angles here.
>
> The easy part of CPUID work is to expose it to PV and HVM guests, and
> this has yet to be done but shouldn't be hard.
>
> The more complicated part, which needs the toolstack CPUID modifications
> I want to do, is to be able to fake up UMIP to HVM guests on non-UMIP
> capable hardware.

Thanks, Andrew.
Our expectation is to get this feature into xen upstream in this year. 
Considering the
xen release schedule, I guess it means to get the easy part accepted in 
xen 4.10.

So I wonder, do you have plan to do this? Any assistance needed from 
Intel? :)

Yu

> ~Andrew
>
> _______________________________________________
> 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