Re: [Xen-devel] [PATCH v3 2/2] x86/Intel: virtualize support for cpuid faulting

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Oct 14, 2016 at 12:47:36PM -0700, Kyle Huey wrote:
> On HVM guests, the cpuid triggers a vm exit, so we can check the emulated
> faulting state in vmx_do_cpuid and inject a GP(0) if CPL > 0. Notably no
> hardware support for faulting on cpuid is necessary to emulate support with an
> HVM guest.
> 
> On PV guests, hardware support is required so that userspace cpuid will trap
> to xen. Xen already enables cpuid faulting on supported CPUs for pv guests 
> (that
> aren't the control domain, see the comment in intel_ctxt_switch_levelling).
> Every PV guest cpuid will trap via a GP(0) to emulate_privileged_op (via
> do_general_protection). Once there we simply decline to emulate cpuid if the
> CPL > 0 and faulting is enabled, leaving the GP(0) for the guest kernel to
> handle.
> 
> Signed-off-by: Kyle Huey <khuey@xxxxxxxxxxxx>

Andrew expressed the desire of taking this patch into 4.8. After reading
the description and code in detail, I think this patch falls into the
"nice-to-have" category.

The main risk here is this patch doesn't have architecturally correct
behaviour. I would like to see an ack or review from VT maintainers to
make this patch eligible for acceptance.

Another thing to consider is timing. We plan to cut RC3 before Friday
this week, so if this patch can be acked and becomes part of RC3 I'm
fine with applying it. If not, we shall revisit the situation when it is
acked.

Wei.

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