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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.10.16 at 09:34, <andrew.cooper3@xxxxxxxxxx> wrote:
> 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.

Oh, of course. Sorry for the noise. But then the check added to
emulate_privileged_op() fails to honor (virtual) CPL afaict.

Jan


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