linux-kernel - Re: [PATCH] X86: MM: Add PAT Type write-through in combination with mtrr

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Date:	Sun, 27 Oct 2013 14:34:01 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andreas Werner <wernerandy@....de>
Cc:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, dave@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] X86: MM: Add PAT Type write-through in combination with
 mtrr

On Sun, Oct 27, 2013 at 01:55:25PM +0100, Andreas Werner wrote:
> This patch adds the Write-through memory type in combination with mtrr.
> If you call ioremap_cache to request cachable memory (write-back) the
> function tries to set the PAT to write-back only if the mtrr setting of
> the requested region is also marked as Write-Back.
> If the mttr regions are marked e.g. as Write-through or with other
> types, the function will always return UC- memory.
> 
> If you check the Intel document " IA-32 SDM vol 3a table Effective
> Memory Type", there
> are many other combinations possible.
> 
> This patch will only add the following combination:
>   PAT=Write-Back + MTRR=Write-Through = Effective Memory of
> Write-Through

And yet the code below returns WB for WT MTRR type which can't be
right since having a write-through mapping cannot be compatible with
write-back as the last caches writes into the cache instead of writing
them through to memory.

Why do you even care about WT? Are you trying to fix a bug or what?

> Tested on - Intel (R) Atom E680 (Tunnel Creek)
>           - Intel (R) Core(TM)2 Duo
> 
> Signed-off-by: Andreas Werner <wernerandy@....de>
> ---
>  arch/x86/mm/pat.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
> index 6574388..9cfe107 100644
> --- a/arch/x86/mm/pat.c
> +++ b/arch/x86/mm/pat.c
> @@ -149,10 +149,15 @@ static unsigned long pat_x_mtrr_type(u64 start, u64 end, unsigned long req_type)
>  		u8 mtrr_type;
>  
>  		mtrr_type = mtrr_type_lookup(start, end);
> -		if (mtrr_type != MTRR_TYPE_WRBACK)
> -			return _PAGE_CACHE_UC_MINUS;
>  
> -		return _PAGE_CACHE_WB;
> +		switch (mtrr_type) {
> +		case MTRR_TYPE_WRBACK:
> +		case MTRR_TYPE_WRTHROUGH:
> +		  return _PAGE_CACHE_WB;
> +
> +		default:
> +		  return _PAGE_CACHE_UC_MINUS;
> +		}
>  	}
>  
>  	return req_type;

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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