Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 14 Sep 2016 11:52:15 -0700 | Subject | Re: [PATCH] prctl,x86 Add PR_[GET|SET]_CPUID for controlling the CPUID instruction. |
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On Tue, Sep 13, 2016 at 11:13 PM, Kyle Huey <me@kylehuey.com> wrote: > On Mon, Sep 12, 2016 at 9:56 AM, Andy Lutomirski <luto@amacapital.net> 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?
--Andy
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