linux-kernel - Re: [PATCH 1/2] x86: Fix pXd_flags() to handle _PAGE_PAT

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Date:	Fri, 10 Jul 2015 05:57:07 +0200
From:	Juergen Gross <jgross@...e.com>
To:	Toshi Kani <toshi.kani@...com>, hpa@...or.com, tglx@...utronix.de,
	mingo@...hat.com
CC:	akpm@...ux-foundation.org, bp@...en8.de, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	konrad.wilk@...cle.com, elliott@...com
Subject: Re: [PATCH 1/2] x86: Fix pXd_flags() to handle _PAGE_PAT_LARGE

On 07/09/2015 07:03 PM, Toshi Kani wrote:
> The PAT bit gets relocated to bit 12 when PUD and PMD mappings are
> used.  This bit 12, however, is not covered by PTE_FLAGS_MASK, which
> is corrently used for masking the flag bits for all cases.
>
> Fix pud_flags() and pmd_flags() to cover the PAT bit, _PAGE_PAT_LARGE,
> when they are used to map a large page with _PAGE_PSE set.
>
> Signed-off-by: Toshi Kani <toshi.kani@...com>
> Cc: Juergen Gross <jgross@...e.com>
> Cc: Konrad Wilk <konrad.wilk@...cle.com>
> Cc: Robert Elliott <elliott@...com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> ---
>   arch/x86/include/asm/pgtable_types.h |   16 +++++++++++++---
>   1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index 13f310b..caaf45c 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -212,9 +212,13 @@ enum page_cache_mode {
>   /* PTE_PFN_MASK extracts the PFN from a (pte|pmd|pud|pgd)val_t */
>   #define PTE_PFN_MASK		((pteval_t)PHYSICAL_PAGE_MASK)
>
> -/* PTE_FLAGS_MASK extracts the flags from a (pte|pmd|pud|pgd)val_t */
> +/* Extracts the flags from a (pte|pmd|pud|pgd)val_t of a 4KB page */
>   #define PTE_FLAGS_MASK		(~PTE_PFN_MASK)
>
> +/* Extracts the flags from a (pmd|pud)val_t of a (1GB|2MB) page */
> +#define PMD_FLAGS_MASK_LARGE	((~PTE_PFN_MASK) | _PAGE_PAT_LARGE)
> +#define PUD_FLAGS_MASK_LARGE	((~PTE_PFN_MASK) | _PAGE_PAT_LARGE)
> +
>   typedef struct pgprot { pgprotval_t pgprot; } pgprot_t;
>
>   typedef struct { pgdval_t pgd; } pgd_t;
> @@ -278,12 +282,18 @@ static inline pmdval_t native_pmd_val(pmd_t pmd)
>
>   static inline pudval_t pud_flags(pud_t pud)
>   {
> -	return native_pud_val(pud) & PTE_FLAGS_MASK;
> +	if (native_pud_val(pud) & _PAGE_PSE)
> +		return native_pud_val(pud) & PUD_FLAGS_MASK_LARGE;
> +	else
> +		return native_pud_val(pud) & PTE_FLAGS_MASK;
>   }
>
>   static inline pmdval_t pmd_flags(pmd_t pmd)
>   {
> -	return native_pmd_val(pmd) & PTE_FLAGS_MASK;
> +	if (native_pmd_val(pmd) & _PAGE_PSE)
> +		return native_pmd_val(pmd) & PMD_FLAGS_MASK_LARGE;
> +	else
> +		return native_pmd_val(pmd) & PTE_FLAGS_MASK;
>   }

Hmm, I think this covers only half of the problem. pud_pfn() and
pmd_pfn() will return wrong results for large pages with PAT bit
set as well.

I'd rather use something like:

static inline unsigned long pmd_pfn_mask(pmd_t pmd)
{
	if (pmd_large(pmd))
		return PMD_PAGE_MASK & PHYSICAL_PAGE_MASK;
	else
		return PTE_PFN_MASK;
}

static inline unsigned long pmd_flags_mask(pmd_t pmd)
{
	if (pmd_large(pmd))
		return ~(PMD_PAGE_MASK & PHYSICAL_PAGE_MASK);
	else
		return ~PTE_PFN_MASK;
}

static inline unsigned long pmd_pfn(pmd_t pmd)
{
         return (pmd_val(pmd) & pmd_pfn_mask(pmd)) >> PAGE_SHIFT;
}

static inline pmdval_t pmd_flags(pmd_t pmd)
{
	return native_pmd_val(pmd) & ~pmd_flags_mask(pmd);
}


Juergen
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