rdmsr from userspace

[Kostik Belousov]

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.

I am interested why Stanislav still did not submitted it for inclusion into
the base still. Maybe, some other reasons exist.
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