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

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Date:	Fri, 22 Aug 2014 07:25:35 +0200
From:	Juergen Gross <jgross@...e.com>
To:	Toshi Kani <toshi.kani@...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
Subject: Re: [PATCH RFC 1/3] x86: Make page cache mode a real type

On 08/22/2014 12:09 AM, Toshi Kani wrote:
> On Tue, 2014-08-19 at 15:25 +0200, jgross@...e.com wrote:
>> From: Juergen Gross <jgross@...e.com>
>>
>> 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.
>
> One more comment below..
>
>> diff --git a/arch/x86/include/asm/cacheflush.h b/arch/x86/include/asm/cacheflush.h
>   :
>> -static inline void set_page_memtype(struct page *pg, unsigned long memtype)
>> +static inline void set_page_memtype(struct page *pg,
>> +				    enum page_cache_mode memtype)
>>   {
>>   	unsigned long memtype_flags = _PGMT_DEFAULT;
>>   	unsigned long old_flags;
>>   	unsigned long new_flags;
>>
>>   	switch (memtype) {
>> -	case _PAGE_CACHE_WC:
>> +	case _PAGE_CACHE_MODE_WC:
>>   		memtype_flags = _PGMT_WC;
>>   		break;
>> -	case _PAGE_CACHE_UC_MINUS:
>> +	case _PAGE_CACHE_MODE_UC_MINUS:
>>   		memtype_flags = _PGMT_UC_MINUS;
>>   		break;
>> -	case _PAGE_CACHE_WB:
>> +	case _PAGE_CACHE_MODE_WB:
>> +	default:
>>   		memtype_flags = _PGMT_WB;
>>   		break;
>>   	}
>
> Adding the "default" case handled as _PGMT_WB is not correct here.
> free_ram_pages_type() calls set_page_memtype() with -1, which needs to
> be set to _PGMT_DEFAULT.

It says so in the comment above. I'll correct it, thanks.

Juergen

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