On 09/14/2016 02:52 PM, Andy Lutomirski wrote: > On Tue, Sep 13, 2016 at 11:13 PM, Kyle Huey <[email protected]> wrote: >> On Mon, Sep 12, 2016 at 9:56 AM, Andy Lutomirski <[email protected]> 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. -boris
