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

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

On 4/19/2017 7:19 PM, Jan Beulich wrote:
> > > > 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).
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
>
>
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