Re: [PATCH v5 6/6] mtrr, mm, x86: Enhance MTRR checks for KVA huge page mapping

[Ingo Molnar]

* Borislav Petkov <bp@xxxxxxxxx> wrote:

> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -566,19 +566,28 @@ void native_set_fixmap(enum fixed_addresses idx, phys_addr_t phys,
>  /**
>   * pud_set_huge - setup kernel PUD mapping
>   *
> - * MTRR can override PAT memory types with 4KiB granularity.  Therefore,
> - * this function does not set up a huge page when the range is covered
> - * by a non-WB type of MTRR.  MTRR_TYPE_INVALID indicates that MTRR are
> - * disabled.
> + * MTRRs can override PAT memory types with 4KiB granularity. Therefore, this
> + * function sets up a huge page only if any of the following conditions are met:
> + *
> + * - MTRRs are disabled, or
> + *
> + * - MTRRs are enabled and the range is completely covered by a single MTRR, or
> + *
> + * - MTRRs are enabled and the range is not completely covered by a single MTRR
> + *   but the memory type of the range is WB, even if covered by multiple MTRRs.
> + *
> + * Callers should try to decrease page size (1GB -> 2MB -> 4K) if the bigger
> + * page mapping attempt fails.

This comment should explain why it's ok in the WB case.

Also, the phrase 'the memory type of the range' is ambiguous: it might 
mean the partial MTRR's, or the memory type specified via PAT by the 
huge-pmd entry.

Thanks,

	Ingo
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