Re: 2.6.26, PAT and AMD family 6

[H. Peter Anvin]

Linus Torvalds wrote:
> On Wed, 7 May 2008, H. Peter Anvin wrote:
> > > What a lovely way of syncing reality with your definitions. The kernel
> > > _does_ see that my CPU features PAT, it just refuses to use it because
> > > it doesn't trust it enough. Vital difference. Maybe not to the kernel,
> > > but definitely to me, the user. /proc/cpuinfo is a user interface.
> > No, /proc/cpuinfo is informing the user about the kernel's view of the CPU.
> > It has always been the "cooked" view of CPUID, whether or not you like it.
>
> Indeed. If people want to know what the CPU itself reports, they should 
> just use the "cpuid" instruction directly. The /proc/cpuinfo file has 
> been a window into the kernels notion of what is going on, and disabling 
> kernel features have disabled the bits in that file as appropriate (eg 
> "clearcpuid=xyzzy" and "mem=nopentium" etc).
>
> Of course, the *common* case is that the two will match 100% in features. 
> But the kernel view has some of its very own set of CPU features too.
>
> And it is possible that the "PAT as the kernel sees it" should be a 
> *separate* bit from "PAT as the cpu itself reports to support it". We do 
> that for quite a lot of the "synthesized" features, eg there is the "TSC" 
> feature as the CPU reports it, and then we have the "CONSTANT_TSC" feature 
> that is the kernel version of it that says that we have a TSC _and_ it 
> runs at a constant rate too (or the "UP" bit, or the "Mfence synchronizes 
> RDTSC" bit etc etc).
>
> So it's possible that we should do the same thing for PAT - and allow 
> people to see the CPU view _and_ the kernel view as two separate issues. 
> In many ways that would be the logical thing to do. Hmm?

Well, there *is* support for that - all the raw information is there in 
/dev/cpu/*/cpuid.  There are other reasons why /proc/cpuinfo is the 
wrong interface to try to get the "real" CPUID information - we only 
report CPU features that the kernel knows about; bits we don't, if we 
can decode them at all, we just don't show.

	-hpa
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