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

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Message-ID: <47F11ED3.40803@tungstengraphics.com>
Date:	Mon, 31 Mar 2008 19:26:43 +0200
From:	Thomas Hellström <thomas@...gstengraphics.com>
To:	Arjan van de Ven <arjan@...ux.intel.com>
CC:	Andi Kleen <andi@...stfloor.org>, Dave Airlie <airlied@...hat.com>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com
Subject: Re: [PATCH] x86: create array based interface to change page attribute

Arjan van de Ven wrote:
> Thomas Hellström wrote:
>> Arjan van de Ven wrote:
>>> Thomas Hellström wrote:
>>>
>>>> Let me rehprase. Not really time-critical but it is of some 
>>>> importance that CPA is done quickly.
>>>> We're dealing with the tradeoff of reading from uncached device memory 
>>>
>>> uncached or write combining ?
>> The user-space mappings (the ones that we really use) are usually 
>> write-combined, whereas the kernel mappings are uncached. (I think 
>> this is OK since both mapping types implies no cache coherency). 
>
> This is not officially allowed and may tripple fault your cpu..
> To comply with the spec one needs to have ALL mappings the same 
> unfortunately.
> (And yes, this is a hard problem)
>
Hmm...

Given this problem, the previously mentioned use-case, and the fact that 
we mostly really use user-space mappings,
Is there a possibility we could add the following functions to Dave's 
patch (provided they would work as intended, of course, namely 
invalidate / bring back the kernel mapping).
Then we can set up a pool of unmapped pages, avoid frequent use of CPA 
and everybody's happy.

int set_memory_array_np(unsigned long *addr, int addrinarray)
{
    return change_page_attr_clr(addr, addrinarray,
                    __pgprot(_PAGE_PRESENT  |  _PAGE_RW), 1);
}
EXPORT_SYMBOL(set_memory_array_np);

int set_memory_array_p(unsigned long *addr, int addrinarray)
{
    return change_page_attr_set(addr, addrinarray,
                    __pgprot(_PAGE_PRESENT  |  _PAGE_RW), 1);
}
EXPORT_SYMBOL(set_memory_array_p);

/Thomas



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