rdmsr from userspace

[Rui Paulo]

Kostik Belousov wrote:
> On Sat, May 17, 2008 at 06:26:01PM +0100, Rui Paulo wrote:
>> Andriy Gapon wrote:
>>> on 17/05/2008 18:37 Rui Paulo said the following:
>>>> Andriy Gapon wrote:
>>>>> It seems that rdmsr instruction can be executed only at the highest 
>>>>> privilege level and thus is not permitted from userland. Maybe we 
>>>>> should provide something like Linux /dev/cpu/msr?
>>>>> I don't like interface of that device, I think that ioctl approach 
>>>>> would be preferable in this case.
>>>>> Something like create /dev/cpuN and allow some ioctls on it: 
>>>>> ioctl(cpu_fd, CPU_RDMSR, arg).
>>>>> What do you think?
>>>>>
>>>> While I think this (devcpu) is good for testing and development, I 
>>>> prefer having a device driver to handle that specific MSR than a 
>>>> generic /dev/cpuN where you can issue MSRs.
>>>> Both for security and reliability reasons.
>>> What about /dev/pci, /dev/io? Aren't they a precedent?
>> They are, but, IMHO, we should no longer continue to create this type of 
>> interfaces.
> 
> Why ? Are developers some kind of the second-class users ?
> 
> I would have no opinion on providing /dev/cpu by the loadable module, not
> compiled into GENERIC. But the interface itself is useful at least for
> three things:
> - CPU identification (see x86info or whatever it is called);
> - CPU tweaking for bugs workaround without patching the kernel;
> - updating the CPU microcode.
> None of these is limited to the developers only.

Input validation is my main concern here. Regarding to your two last 
points, I would prefer to have a microcode driver than a microcode 
userland utility that relies on devcpu.

Regards,
-- 
Rui Paulo