linux-kernel - Re: [PATCH] x86: create array based interface to change page attribute

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Date:	Mon, 31 Mar 2008 09:55:20 +0200
From:	Thomas Hellström <thomas@...gstengraphics.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Dave Airlie <airlied@...hat.com>, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, mingo@...hat.com, arjan@...ux.intel.com
Subject: Re: [PATCH] x86: create array based interface to change page attribute

Andi Kleen wrote:
> Dave Airlie <airlied@...hat.com> writes:
>   
>>  
>> +#define CPA_FLUSHTLB 1
>> +#define CPA_ARRAY 2
>>     
>
> I don't think CPA_ARRAY should be a separate case. Rather single
> page flushing should be an array with only a single entry. pageattr
> is already very complex, no need to make add more special cases.
>   
>> +
>> +		/*
>> +		 * Only flush present addresses:
>> +		 */
>> +		if (pte && (pte_val(*pte) & _PAGE_PRESENT))
>> +			clflush_cache_range((void *) *addr, PAGE_SIZE);
>>     
>
> Also it is doubtful clflush really makes sense on a large array. Just
> doing wbinvd might be faster then. Or perhaps better supporting Self-Snoop
> should be revisited, that would at least eliminate it on most Intel 
> CPUs.
>
>   
I agree that wbinvd() seems to be faster on large arrays on the 
processors I've tested. But isn't there a severe latency problem with 
that instruction, that makes people really want to avoid it in all 
possible cases?

Also I think we need to clarify the semantics of the c_p_a 
functionality. Right now both AGP and DRM relies on c_p_a  doing an 
explicit cache flush. Otherwise the data won't appear on the device side 
of the aperture.
If we use self-snoop, the AGP and DRM drivers can't rely on this flush 
being performed, and they have to do the flush themselves, and for 
non-self-snooping processors, the flush needs to be done twice?

/Thomas

> -Andi
>   



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