Re: PAT 64b: Basic PAT implementation

Re: PAT 64b: Basic PAT implementation

[Daniel J Blueman]

On 14 Dec, 00:50, Andi Kleen <a...@muc.de> wrote:
> > +void __cpuinit pat_init(void)
> > +{
> > +  /* Set PWT+PCD to Write-Combining. All other bits stay the same */
> > +  if (cpu_has_pat) {
>
> All the old CPUs (PPro etc.) with known PAT bugs need to clear this flag
> now in their CPU init functions. It is fine to be aggressive there
> because these old systems have lived so long without PAT they can do
> so forever. So perhaps it's best to just white list it only for newer
> CPUs on the Intel side at least.

> Another problem is that there are some popular modules (ATI, Nvidia for once)
> who reprogram the PAT registers on their own, likely different. Need some way to detect
> that case I guess, otherwise lots of users will see strange malfunctions.
> Maybe recheck after module load?

This may not be as big problem as thought, since sane and at least one
vendor driver (Quadrics QsNetII) searches the PAT slots for a WC entry
- where this has already been setup by the kernel, it'll use the right
one.

> > +                   |||
> > +             000 WB         default
> > +             010 UC_MINUS   _PAGE_PCD
> > +             011 WC         _PAGE_WC
> > +             PAT bit unused */
> > +          pat = PAT(0,WB) | PAT(1,WT) | PAT(2,UC_MINUS) | PAT(3,WC) |
> > +                PAT(4,WB) | PAT(5,WT) | PAT(6,UC_MINUS) | PAT(7,WC);
> > +          rdmsrl(MSR_IA32_CR_PAT, boot_pat_state);
> > +          wrmsrl(MSR_IA32_CR_PAT, pat);
> > +          __flush_tlb_all();
> > +          asm volatile("wbinvd");
>
> Have you double checked this is the full procedure from the manual? iirc there
> were some steps missing.
>
> -Andi
-- 
Daniel J Blueman