Enabling PAT in 2.6.26.x

Enabling PAT in 2.6.26.x

[Chris Rankin]

Hi,

I see that the PAT support in 2.6.26.x depends on the CPUID information, and that it is currently disabled for some older model Pentium IIIs:

processor	: 0
vendor_id	: GenuineIntel
cpu family	: 6
model		: 7
model name	: Pentium III (Katmai)
stepping	: 3
cpu MHz		: 498.862
cache size	: 512 KB
fdiv_bug	: no
hlt_bug		: no
f00f_bug	: no
coma_bug	: no
fpu		: yes
fpu_exception	: yes
cpuid level	: 2
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 mmx fxsr sse
bogomips	: 998.80
clflush size	: 32

I suppose that I could hack the kernel to allow PAT to be enabled for this model CPU, but I'm not sure how meaningful the results would be. Does PAT support either work immediately or not at all? Or are any errors likely to be more subtle?

Basically, is there any reliable way of confirming whether PAT can be enabled for the CPU, please?

Thanks,
Chris



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Re: Enabling PAT in 2.6.26.x

[Arjan van de Ven]

On Sat, 2 Aug 2008 11:16:35 -0700 (PDT)
Chris Rankin <rankincj@yahoo.com> wrote:

> Hi,
> 
> I see that the PAT support in 2.6.26.x depends on the CPUID
> information, and that it is currently disabled for some older model
> Pentium IIIs:
> 
> 
> I suppose that I could hack the kernel to allow PAT to be enabled for
> this model CPU, but I'm not sure how meaningful the results would be.
> Does PAT support either work immediately or not at all? Or are any
> errors likely to be more subtle?

errors are a LOT more subtle sadly, things like "once every month a
spontaneous reboot happens"

> 
> Basically, is there any reliable way of confirming whether PAT can be
> enabled for the CPU, please?

It'll come down to someone reading the CPU errata sheet to see if
there's known issues.

For Intel cpus.... I'll nominate Venki for this (he already did this
for many models)


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