Re: [Xen-devel] [PATCH RFC] x86/Intel: virtualize support for cpuid faulting

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 04/10/2016 08:25, Jan Beulich wrote:
>>>> On 04.10.16 at 00:38, <me@xxxxxxxxxxxx> wrote:
>> rr (http://rr-project.org/), a Linux userspace record-and-replay reverse-
>> execution debugger, would like to trap and emulate the CPUID instruction.
>> This would allow us to a) mask away certain hardware features that rr does
>> not support (e.g. RDRAND) and b) enable trace portability across machines
>> by providing constant results. Patches for support in the Linux kernel are in
>> flight, and we'd like to be able to use this feature on virtualized Linux
>> instances as well.
>>
>> On HVM guests, the cpuid triggers a vm exit, so we can check the emulated
>> faulting state in vmx_do_cpuid and inject a GP(0) if CPL > 0.
> Why for CPL > 0 only? I don't think hardware CPUID faulting is CPL
> sensitive.

CPUID Faulting is CPL sensitive.  Otherwise Xen have a hard time
executing cpuid instructions itself.

~Andrew

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