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

[Kyle Huey <me@xxxxxxxxxxxx>]

On Mon, Oct 17, 2016 at 5:32 AM, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
> 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.

Kevin Tian reviewed the patch yesterday, so I think we're just waiting
for a final review from Andrew here.

That said, rr currently does not work in Xen guests due to some PMU
issues that we haven't tracked down yet.  So for us it's not a big
deal if this feature does not make it into 4.8.  I won't be
disappointed if you cut it from 4.8 to reduce technical risk.

- Kyle

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel