Re: [PATCH v2 2/3] x86 Test and expose CPUID faulting capabilities in /proc/cpuinfo

[Kyle Huey]

On Wed, Sep 14, 2016 at 6:17 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Wed, Sep 14, 2016 at 3:03 PM, Kyle Huey <me@xxxxxxxxxxxx> wrote:
>> On Wed, Sep 14, 2016 at 2:35 PM, Dave Hansen
>> <dave.hansen@xxxxxxxxxxxxxxx> wrote:
>>> On 09/14/2016 02:01 PM, Kyle Huey wrote:
>
>>> Is any of this useful to optimize away at compile-time?  We have config
>>> options for when we're running as a guest, and this seems like a feature
>>> that isn't available when running on bare metal.
>>
>> On the contrary, this is only available when we're on bare metal.
>> Neither Xen nor KVM virtualize CPUID faulting (although KVM correctly
>> suppresses MSR_PLATFORM_INFO's report of support for it).
>
> KVM could easily support this.  If rr starts using it, I think KVM
> *should* add support, possibly even for older CPUs that don't support
> the feature in hardware.
>
> It's too bad that x86 doesn't give us the instruction bytes on a
> fault.  Otherwise we could lazily switch this feature.

We are *very* interested in having KVM and Xen support virtualization
of this feature.  I am planning to work on KVM after I get this series
of patches in :)

- Kyle