rdmsr from userspace

[Rui Paulo]

Kostik Belousov wrote:
> On Sun, May 18, 2008 at 04:47:41PM +0100, Rui Paulo wrote:
>> 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.
> Did you looked at the code ? It does exactly what you described.
> 
> Driver has four basic operations:
> read/write msr
> perform cpu id work
> update microcode.
> 
> The later is done as a whole operation, with the microcode blob supplied
> by the userspace.

Yes, but I still don't like having everything mixed up in one driver. At 
the very least, I would like us to have two drivers. One for the 
microcode update and the other driver for the rest.

I would like to see a microcode update utility (driver + something to 
parse Intel's file aka devcpu-data) in the base system, but not "the 
rest", though.

Regards,
-- 
Rui Paulo