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





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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