Re: [RFC] Cachemap for 2.6.12rc4-mm1. Was Re: [PATCH] enhance x86 MTRR handling

Dave Jones
2005-05-13T23:26:09Z

On Fri, May 13, 2005 at 03:40:34PM -0700, H. Peter Anvin wrote:
> Dave Jones wrote:
>
> >+ switch (boot_cpu_data.x86_vendor) {
> >+ case X86_VENDOR_AMD:
> >+ wrmsr(IA32_CR_PAT, AMD_PAT_31_0, AMD_PAT_63_32);
> >+ atomic_inc(&pat_cpus_enabled);
> >+ break;
> >+ case X86_VENDOR_INTEL:
> >+ wrmsr(IA32_CR_PAT, INTEL_PAT_31_0, INTEL_PAT_63_32);
> >+ atomic_inc(&pat_cpus_enabled);
> >+ break;
> >+ default:
> >+ printk("Unknown vendor in setup_pat()\n");
> >+ }
>
> Drop the vendor check; PAT is a generic x86 feature. If AMD is not
> compatible (see below), then use X86_VENDOR_AMD: and default:.

Done. Does transmeta have PAT btw ? I know newer VIA has it,
but I haven't looked through the docs to double check its
implementation yet.

> >+ /* checks copied from arch/i386/kernel/cpu/mtrr/main.c */
> >+ /* do these only apply to mtrrs or pat as well? */
>
> It would apply to both; the chipset wouldn't even know how it got invoked.

ACK, Comment dropped.

> >+/* Here is the PAT's default layout on ia32 cpus when we are done.
> >+ * PAT0: Write Back
> >+ * PAT1: Write Combine
> >+ * PAT2: Uncached
> >+ * PAT3: Uncacheable
> >+ * PAT4: Write Through
> >+ * PAT5: Write Protect
> >+ * PAT6: Uncached
> >+ * PAT7: Uncacheable
>
> Bad move. Some (Intel) processors drop the top bit, so it's much better
> to pick the protection methods one cares about (usually WB, WC, UC) and
> stick them in the first four, then duplicate the whole thing in the
> second half.

Noted.

> >+ * Note: On Athlon cpus PAT2/PAT3 & PAT6/PAT7 are both Uncacheable since
> >+ * there is no uncached type.
> If one sets the PAT to "uncached", does one get the same function as
> "uncachable"?

AIUI, only as long as we don't have an MTRR covering the same range marked WC.
It seems to be the only thing I could find documenting the differences
between 'uncached' and 'uncacheable' in this context.
Though I've only looked through the Intel & AMD K8 docs, I don't have
the K7 ones to hand.

Dave