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SubjectRe: [PATCH] X86: don't report PAT on CPUs that don't support it

On Tue, 18 Apr 2017, H. Peter Anvin wrote:

> On 04/18/17 12:07, Mikulas Patocka wrote:
> > In the file arch/x86/mm/pat.c, there's a variable __pat_enabled. The
> > variable is set to 1 by default and the function pat_init() sets
> > __pat_enabled to 0 if the CPU doesn't support PAT.
> >
> > However, on AMD K6-3 CPU, the processor initialization code never calls
> > pat_init() and so __pat_enabled stays 1 and the function pat_enabled()
> > returns true, even though the K6-3 CPU doesn't support PAT.
> >
> > The result of this bug is that this warning is produced when attemting to
> > start the Xserver and the Xserver doesn't start (fork() returns ENOMEM).
> > Another symptom of this bug is that the framebuffer driver doesn't set the
> > K6-3 MTRR registers.
> >
> > This patch changes pat_enabled() so that it returns true only if pat
> > initialization was actually done.
> >
> > Also, I changed boot_cpu_has(X86_FEATURE_PAT) to
> > this_cpu_has(X86_FEATURE_PAT) in pat_ap_init, so that we check the PAT
> > feature on the processor that is being initialized.
> >
> I'm thinking it would be better to replace __pat_enabled with
> static_cpu_has(X86_FEATURE_PAT) and disable the feature bit if the user
> has disabled it on the command line, just as we do with other features.
> -hpa

If MTRR initialization fails for whatever reason, then pat_init() won't be
called and the kernel would mistakenly believe that PAT is working
(because there would be no one to clear X86_FEATURE_PAT).

I think that pat should be reported only if pat_init() is called and

Another strange thing: pat_disable() calls init_cache_modes() - but since
pat_disable() may not be called at all, it is possible that
init_cache_modes() is also not called at all. It doesn't produce any
visible misbehavior on my machine, but it doesn't seem right - we should
not call init_cache_modes() from pat_disable() and do the initialization
elsewhere, where it is always called.


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