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

From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Kyle Huey <me@kylehuey.com>, Wei Liu <wei.liu2@citrix.com>
Cc: Robert O'Callahan <robert@ocallahan.org>,
	Kevin Tian <kevin.tian@intel.com>,
	Jan Beulich <JBeulich@suse.com>,
	Jun Nakajima <jun.nakajima@intel.com>,
	xen-devel@lists.xen.org
Subject: Re: [PATCH v3 2/2] x86/Intel: virtualize support for cpuid faulting
Date: Thu, 20 Oct 2016 08:56:19 +0100	[thread overview]
Message-ID: <2cc2636e-f8b0-f90b-a460-9c664e1a748a@citrix.com> (raw)
In-Reply-To: <CAP045Aqh7XB-QVZ8UTqscBuY+THn6gzGHBwMCREQE6USA0pvZA@mail.gmail.com>

On 20/10/2016 06:10, Kyle Huey wrote:
> On Mon, Oct 17, 2016 at 5:32 AM, Wei Liu <wei.liu2@citrix.com> 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@kylehuey.com>
>> 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.

Ah - I am just waiting for your final respin with the comments so far
addressed.

>
> That said, rr currently does not work in Xen guests due to some PMU
> issues that we haven't tracked down yet.

Is this RR trying to use vPMU and it not functioning, or not
specifically trying to use PMU facilities and getting stuck anyway?

>   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.

From my point of view, its a small feature with working code and a
comprehensive test case ready to go straight into regression testing. 
This makes it the least risky feature going.

~Andrew

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