Re: [PATCH] prctl,x86 Add PR_[GET|SET]_CPUID for controlling the CPUID instruction.

From: Andy Lutomirski
Date: Wed Sep 14 2016 - 15:37:09 EST

On Wed, Sep 14, 2016 at 12:28 PM, Andrew Cooper
<andrew.cooper3@xxxxxxxxxx> wrote:
> On 14/09/2016 20:23, Boris Ostrovsky wrote:
>> On 09/14/2016 02:52 PM, Andy Lutomirski wrote:
>>> On Tue, Sep 13, 2016 at 11:13 PM, Kyle Huey <me@xxxxxxxxxxxx> wrote:
>>>> On Mon, Sep 12, 2016 at 9:56 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>>>> You should explicitly check that, if the
>>>>> feature is set under Xen PV, then the MSR actually works as
>>>>> advertised. This may require talking to the Xen folks to make sure
>>>>> you're testing the right configuration.
>>>> This is interesting. When running under Xen PV the kernel is allowed
>>>> to read the real value of MSR_PLATFORM_INFO and see that CPUID
>>>> faulting is supported. But as you suggested, writing to
>>>> MSR_MISC_FEATURES_ENABLES doesn't actually enable CPUID faulting, at
>>>> least not in any way that works.
>>>> It's not obvious to me how to test this, because when this feature
>>>> works, CPUID only faults in userspace, not in the kernel. Is there
>>>> existing code somewhere that runs tests like this in userspace?
>>> Andrew, Boris: should we expect Xen PV to do anything sensible when we
>>> write to MSR_PLATFORM_INFO to turn on CPUID faulting? Should the Xen
>>> PV rdmsr hooks or perhaps the hypervisor mask out the feature if it
>>> isn't going to be supported?
>> The hypervisor uses CPUID faulting so we shouldn't advertise this
>> feature to guests.
> In the case that the hardware has faulting, or for any HVM guest, the
> extra cost to making the feature available to the guest is a single
> conditional test in the cpuid path. This is about as close to zero as a
> feature gets. We really should be offering the feature to guests, and
> have it actually working. The issue here is that it is leaking when we
> weren't intending to offer it.

As long as Xen can fix this one way or the other in reasonably short
order, I think I'm okay with having Linux incorrectly think it works
on old Xen hypervisors.