Linux Virtualization - Sep 2014 - Standardizing an MSR or other hypercall to get an RNG seed?

On Sep 19, 2014 9:40 AM, "H. Peter Anvin" <hpa at zytor.com>
wrote:
>
> On 09/19/2014 09:14 AM, Nakajima, Jun wrote:
> >
> > I slept on it, and I think using the CPUID instruction alone would be
> > simple and efficient:
> > - We have a huge space for CPUID leaves
> > - CPUID also works for user-level
> > - It can take an additional 32-bit parameter (ECX), and returns 4
> > 32-bit values (EAX, EBX, ECX, and EDX).  RDMSR, for example, returns a
> > 64-bit value.
> >
> > Basically we can use it to implement a hypercall (rather than VMCALL).
> >
> > For example,
> > - CPUID 0x48000001.EAX would return the feature presence (e.g. in
> > EBX), and the result in EDX:EAX (if present) at the same time, or
> > - CPUID 0x48000001.EAX would return the feature presence only, and
> > CPUID 0x48000002.EAX (acts like a hypercall) returns up to 4 32-bit
> > values.
> >
>
> There is a huge disadvantage to the fact that CPUID is a user space
> instruction, though.
We can always make cpuid on the leaf in question return all zeros if CPL > 0.
>
>         -hpa
>

On 09/19/2014 10:21 AM, Andy Lutomirski wrote:
>>
>> There is a huge disadvantage to the fact that CPUID is a user space
>> instruction, though.
> 
> We can always make cpuid on the leaf in question return all zeros if CPL
> 0.
> 
Not sure that is better...

	-hpa

On Fri, Sep 19, 2014 at 10:36 AM, H. Peter Anvin <hpa at zytor.com>
wrote:
> On 09/19/2014 10:21 AM, Andy Lutomirski wrote:
>>>
>>> There is a huge disadvantage to the fact that CPUID is a user space
>>> instruction, though.
>>
>> We can always make cpuid on the leaf in question return all zeros if
CPL > 0.
>>
>
> Not sure that is better...
It's better than #GP...

This is why I prefer rdmsr: the privilege semantics are already
appropriate.  Also, I wouldn't be surprised if shoehorning
non-constant results into cpuid implementations might be awkward for
some hypervisors.

--Andy