On 4/19/2017 5:18 PM, Jan Beulich wrote:
On 19.04.17 at 10:48, <yu.c.zh...@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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