linux-kernel - Re: [PATCH V2 1/3] x86: Make page cache mode a real type

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Date:	Tue, 09 Sep 2014 11:24:52 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Juergen Gross <jgross@...e.com>
Cc:	stefan.bader@...onical.com, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xensource.com, konrad.wilk@...cle.com,
	ville.syrjala@...ux.intel.com, hpa@...or.com, x86@...nel.org,
	jbeulich@...e.com, david.vrabel@...rix.com
Subject: Re: [PATCH V2 1/3] x86: Make page cache mode a real type

On Mon, 2014-09-08 at 10:01 +0200, Juergen Gross wrote:
> At the moment there are a lot of places that handle setting or getting
> the page cache mode by treating the pgprot bits equal to the cache mode.
> This is only true because there are a lot of assumptions about the setup
> of the PAT MSR. Otherwise the cache type needs to get translated into
> pgprot bits and vice versa.
> 
> This patch tries to prepare for that by introducing a seperate type
> for the cache mode and adding functions to translate between those and
> pgprot values.
> 
> To avoid too much performance penalty the translation between cache mode
> and pgprot values is done via tables which contain the relevant
> information.  Write-back cache mode is hard-wired to be 0, all other
> modes are configurable via those tables. For large pages there are
> translation functions as the PAT bit is located at different positions
> in the ptes of 4k and large pages.
> 
> Signed-off-by: Stefan Bader <stefan.bader@...onical.com>
> Signed-off-by: Juergen Gross <jgross@...e.com>

I have two minor comments (which I should have caught before)... Other
wise, the changes look good to me.

Reviewed-by: Toshi Kani <toshi.kani@...com>


>  /*
> diff --git a/arch/x86/include/asm/fb.h b/arch/x86/include/asm/fb.h
> index 2519d06..0902a50 100644
> --- a/arch/x86/include/asm/fb.h
> +++ b/arch/x86/include/asm/fb.h
> @@ -8,8 +8,12 @@
>  static inline void fb_pgprotect(struct file *file, struct vm_area_struct *vma,
>  				unsigned long off)
>  {
> +	unsigned long prot;
> +
> +	prot = pgprot_val(vma->vm_page_prot) & ~_PAGE_CACHE_MASK;
>  	if (boot_cpu_data.x86 > 3)
> -		pgprot_val(vma->vm_page_prot) |= _PAGE_PCD;
> +		pgprot_val(vma->vm_page_prot) =
> +			prot | cachemode2protval(_PAGE_CACHE_MODE_UC);

This should be _PAGE_CACHE_MODE_UC_MINUS as the original code only sets
the PCD bit.

> diff --git a/drivers/video/fbdev/gbefb.c b/drivers/video/fbdev/gbefb.c
> index 4aa56ba..b1cc7a6 100644
> --- a/drivers/video/fbdev/gbefb.c
> +++ b/drivers/video/fbdev/gbefb.c
> @@ -54,7 +54,8 @@ struct gbefb_par {
>  #endif
>  #endif
>  #ifdef CONFIG_X86
> -#define pgprot_fb(_prot) ((_prot) | _PAGE_PCD)
> +#define pgprot_fb(_prot) (((_prot) & ~_PAGE_CACHE_MASK) |	\
> +			  cachemode2protval(_PAGE_CACHE_MODE_UC))

Ditto. 

Thanks,
-Toshi



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