Re: Support for i386 PATs

[Avi Kivity <avi@argo.co.il>]

Thomas Hellström wrote:
> Hi!
>
> Does anybody have a strong opinion against adding support for
> i386 Page Attribute Tables?
>
> The main benefit would be that one can have write-combining memory
> regions without setting up MTRRs. This will come in handy for a
> device we're working with where the device driver needs to allocate the
> display memory directly from system memory, and it may be difficult to 
> fit
> the mtrr alignment constraints.

An additional benefit if that you can easily run out of mtrrs.  My home 
machine boots with all 8 mtrrs used, leaving none for the G965 
graphics.  I have to compress two mtrrs into one in order to get decent 
performance.

>
> Outline:
> The PAT may be set up at boot time with fixed backwards-compatible
> memory types for the different PAT entries + defines like the following:
>
> #define _PAGE_PAT_WB   xxx
> #define _PAGE_PAT_WT   xxx
> #define _PAGE_PAT_UC0 xxx
> #define _PAGE_PAT_UC1 xxx
> #define _PAGE_PAT_WC   xxx
>
> which can be used in parallel with the old _PAGE_PWT and _PAGE_PCD bits.
>
> The idea is that new memory types, WC for example, will use the pat 
> entries
> 7 downto 4, whereas 0-3 are left to boot setting to maintain backwards 
> compatibility.
>
> Issues:
> 1) The _PAGE_BIT_PAT will be the same as _PAGE_PSE, and _PAGE_PROTNONE.
> As I understand it, _PAGE_PROTNONE is not used when the page is 
> present, so this might not be an issue.
> What about _PAGE_PSE?

pse pages place the pat bit in bit 12, you will need to account for that.



-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.