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

On 09/14/2016 02:52 PM, Andy Lutomirski wrote:

On Tue, Sep 13, 2016 at 11:13 PM, Kyle Huey [off-list ref] wrote:

quoted

On Mon, Sep 12, 2016 at 9:56 AM, Andy Lutomirski [off-list ref] wrote:

quoted

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.


-boris