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

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

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.

--Andy

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