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

[H. Peter Anvin]

On September 14, 2016 6:17:51 PM PDT, 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.
>
>--Andy

You can "always" examine the instruction bytes in memory... have to make sure you properly consider the impact of race conditions though.
-- 
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