Re: about fully UMIP support in Xen

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

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?

Yu


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


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