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

[Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>]

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@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? 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
>
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