Date:	Thu, 29 May 2008 23:51:34 +0200
From:	Rufus & Azrael <rufus-azrael@...ericable.fr>
To:	Venki Pallipadi <venkatesh.pallipadi@...el.com>
CC:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	Linux-kernel Mailing List <linux-kernel@...r.kernel.org>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [2.6.25-git18 => 2.6.26-rc1-git1] Xorg crash with	xf86MapVidMem
 error

Venki Pallipadi wrote:
> On Thu, May 08, 2008 at 05:34:24PM -0700, Pallipadi, Venkatesh wrote:
>    
>>     On Thu, May 08, 2008 at 04:54:44PM -0700, H. Peter Anvin wrote:
>>     >  Venki Pallipadi wrote:
>>     >  >
>>     >  >Also, note that we only look for start while looking at fixed range
>>     MTRRs.
>>     >  >This is not as scary as it seems. We are finding the effective memory
>>     type
>>     >  >by just looking at the start of the address range. We still go through
>>     >  >the PAT reserve free mechanism, once we find the effective memory type
>>     >  >and that list will catch any other users with conflicting type anywhere
>>     >  >in the start to end range. And we will still keep effective type
>>     consistent
>>     >  >across all mappings.
>>     >  >
>>     >
>>     >  So what you're saying here is "it's bogus, but it doesn't really matter
>>     >  anyway?"  Why bother having it at all, then?
>>     >
>>     >  Seriously, if it's not unconditionally correct, then:
>>     >
>>     >  a. it should be *clearly* labelled a heuristic.
>>     >  b. it should be *clearly* explained why having the heuristic is much
>>     >  better than not having anything.
>>     >
>>     >  In this case, neither of those conditions appear to be addressed.
>>
>>     Agree that is has to be called a heuristic. Yes. Not having that will work
>>     may be not as optimially. Having it gives us better starting point when we
>>     start to find a proper memory type for requests, especially when there are
>>     no other overlapping mappings in PAT list.
>>
>>     One of the reason for heristic was to get proper type for /dev/mem maps
>>     for WB
>>     region (ACPI and BIOS area). /dev/mem checks to see the mtrr type of the
>>     start
>>     address and starts with that type for the request. As long as there are no
>>     other conflict in PAT list, we can give WB to this /dev/mem request. Not
>>     having this heuristic we will have to most probably default to UC.
>>
>>     When there are overlapping PAT list entries, the mtrr_lookup does not
>>     matter
>>     as we have to inherit things from those PAT entries or conflicting with
>>     them.
>>
>>     This discussion points to - code is not sufficiently commented and/or
>>     needs
>>     some refactoring. Will look at this afresh tomorrow morning and see
>>     whether
>>     I can some up with some better alternative.
>>
>>      
>
>
> This is an alternate solution to the problem in this thread. The change
> clarifies what is expected out of mtrr_lookup in PAT code and how we handle
> failure.
>
> Thanks,
> Venki
>
>
>
> Alternate patch to clarify the usage of mtrr_lookup() in PAT code, and to make
> PAT code resilient to mtrr lookup problems.
>
> Specifically, pat_x_mtrr_type() is restructured to highlight, under
> what conditions we look for mtrr hint. pat_x_mtrr_type() uses a default
> type when there are any errors in mtrr lookup (still maintaining the pat
> consistency). And, reserve_memtype() highlights its usage ot mtrr_lookup for
> request type of '-1' and also defaults in a sane way on any mtrr lookup failure.
>
> pat.c looks at mtrr type of a range to get a hint on what mapping type
> to request when user/API:
> (1) hasn't specified any type (/dev/mem mapping) and we do not want to take
> performance hit by always mapping UC_MINUS. This will be the case
> for /dev/mem mappings used to map BIOS area or ACPI region which are WB'able.
> In this case, as long as MTRR is not WB, PAT will request UC_MINUS for such
> mappings.
>
> (2) user/API requests WB mapping while in reality MTRR may have UC or WC.
> In this case, PAT can map as WB (without checking MTRR) and still effective
> type will be UC or WC. But, a subsequent request to map same region as
> UC or WC may fail, as the region will get trackked as WB in PAT list. Looking
> at MTRR hint helps us to track based on effective type rather than what user
> requested. Again, here mtrr_lookup is only used as hint and we fallback to
> WB mapping (as requested by user) as default.
>
> In both cases, after using the mtrr hint, we still go through the memtype
> list to make sure there are no inconsistencies among multiple users.
>
>
> Signed-off-by: Venkatesh Pallipadi<venkatesh.pallipadi@...el.com>
> Signed-off-by: Suresh Siddha<suresh.b.siddha@...el.com>
>
> ---
>   arch/x86/mm/pat.c |   49 ++++++++++++++++++++++++-------------------------
>   1 file changed, 24 insertions(+), 25 deletions(-)
>
> Index: linux-2.6/arch/x86/mm/pat.c
> ===================================================================
> --- linux-2.6.orig/arch/x86/mm/pat.c	2008-05-28 13:19:28.000000000 -0700
> +++ linux-2.6/arch/x86/mm/pat.c	2008-05-28 15:18:22.000000000 -0700
> @@ -151,32 +151,33 @@ static int pat_x_mtrr_type(u64 start, u6
>   	unsigned long pat_type;
>   	u8 mtrr_type;
>
> -	mtrr_type = mtrr_type_lookup(start, end);
> -	if (mtrr_type == 0xFF) {		/* MTRR not enabled */
> -		*ret_prot = prot;
> -		return 0;
> -	}
> -	if (mtrr_type == 0xFE) {		/* MTRR match error */
> -		*ret_prot = _PAGE_CACHE_UC;
> -		return -1;
> -	}
> -	if (mtrr_type != MTRR_TYPE_UNCACHABLE&&
> -	    mtrr_type != MTRR_TYPE_WRBACK&&
> -	    mtrr_type != MTRR_TYPE_WRCOMB) {	/* MTRR type unhandled */
> -		*ret_prot = _PAGE_CACHE_UC;
> -		return -1;
> -	}
> -
>   	pat_type = prot&  _PAGE_CACHE_MASK;
>   	prot&= (~_PAGE_CACHE_MASK);
>
> -	/* Currently doing intersection by hand. Optimize it later. */
> +	/*
> +	 * We return the PAT request directly for types where PAT takes
> +	 * precedence with respect to MTRR and for UC_MINUS.
> +	 * Consistency checks with other PAT requests is done later
> +	 * while going through memtype list.
> +	 */
>   	if (pat_type == _PAGE_CACHE_WC) {
>   		*ret_prot = prot | _PAGE_CACHE_WC;
> +		return 0;
>   	} else if (pat_type == _PAGE_CACHE_UC_MINUS) {
>   		*ret_prot = prot | _PAGE_CACHE_UC_MINUS;
> -	} else if (pat_type == _PAGE_CACHE_UC ||
> -	           mtrr_type == MTRR_TYPE_UNCACHABLE) {
> +		return 0;
> +	} else if (pat_type == _PAGE_CACHE_UC) {
> +		*ret_prot = prot | _PAGE_CACHE_UC;
> +		return 0;
> +	}
> +
> +	/*
> +	 * Look for MTRR hint to get the effective type in case where PAT
> +	 * request is for WB.
> +	 */
> +	mtrr_type = mtrr_type_lookup(start, end);
> +
> +	if (mtrr_type == MTRR_TYPE_UNCACHABLE) {
>   		*ret_prot = prot | _PAGE_CACHE_UC;
>   	} else if (mtrr_type == MTRR_TYPE_WRCOMB) {
>   		*ret_prot = prot | _PAGE_CACHE_WC;
> @@ -233,14 +234,12 @@ int reserve_memtype(u64 start, u64 end,
>
>   	if (req_type == -1) {
>   		/*
> -		 * Special case where caller wants to inherit from mtrr or
> -		 * existing pat mapping, defaulting to UC_MINUS in case of
> -		 * no match.
> +		 * Call mtrr_lookup to get the type hint. This is an
> +		 * optimization for /dev/mem mmap'ers into WB memory (BIOS
> +		 * tools and ACPI tools). Use WB request for WB memory and use
> +		 * UC_MINUS otherwise.
>   		 */
>   		u8 mtrr_type = mtrr_type_lookup(start, end);
> -		if (mtrr_type == 0xFE) { /* MTRR match error */
> -			err = -1;
> -		}
>
>   		if (mtrr_type == MTRR_TYPE_WRBACK) {
>   			req_type = _PAGE_CACHE_WB;
>
>
>    
Hi Venki,

I try your patch above and it works fine for my config, as your previous 
patch.

Thanks for your help.

Regards.
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