Re: [PATCH v2 0/8] x86/mtrr: fix handling with PAT but without MTRR

[Juergen Gross]

On 16.02.23 00:22, Linus Torvalds wrote:
> On Wed, Feb 15, 2023 at 12:25 AM Juergen Gross <jgross@xxxxxxxx> wrote:
> > The problem arises in case a large mapping is spanning multiple MTRRs,
> > even if they define the same caching type (uniform is set to 0 in this
> > case).
>
> Oh, I think then you should fix uniform to be 1.
>
> IOW, we should not think "multiple MTRRs" means "non-uniform". Only
> "different actual memory types" should mean non-uniformity.

Thanks for confirmation. I completely agree.

> If I remember correctly, there were good reasons to have overlapping
> MTRR's. In fact, you can generate a single MTRR that described a
> memory ttype that wasn't even contiguous if you had odd memory setups.
>
> Intel definitely defines how overlapping MTRR's work, and "same types
> overlaps" is documented as a real thing.

Yes. And it is handled wrong in current code.

Handling it correctly will require quite some reworking of the code,
which I've already started to work on. I will defer the pud_set_huge()/
pmd_set_huge() modifying patch to after this rework.


Juergen

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