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

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Date:	Wed, 02 Apr 2008 08:59:41 +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:
>
>> But what mappings are there,  immediately after alloc_page(), that 
>> set_memory_np won't catch? 
>
> For example on x86 64bit, the kernel text is mapped (to allow 
> relocatable) in another
> space as well.. and to allow 2Mb tlbs for this, that second mapping is 
> bigger than it strictly needs to be.
> So your alloc_page() could get a page from the free pool that comes 
> from the pages that have a second mapping
> due to this rounding.
> (and more fun, since it's close to frequently accessed memory, the hw 
> prefetchers may actually just decide to pull
> such pages into the cache preemptively)
>
>
>> Drivers relying on set_memory_uc touching all mappings the driver 
>> hasn't set up itself must then have the same problem and needs to be 
>> fixed; referring in particular to agpgart for which driver the old 
>> CPA functionality was once created, IIRC.
>
> "uc" is different than "np"; for "uc" the implementation, if your cpu 
> needs this, will fix
> up the shadow text mappings already today.
>
> for "present" this doesn't make sense, since no cpu needs this.
If this is the checkalias() thingy in x86/pageattr.c, it looks like it 
doesn't handle np different from the uc case. It does obviously skip NX 
bit manipulations, though.

Anyway, a more direct question: If we were to fix whatever's missing for 
the code to fixup the x86-64 shadow text mapping, would you be opposing 
this way to fix the long standing uc/wc aliasing issue, provided we 
don't hit any other problems, like clflush() refusing to run on an NP page?

/Thomas



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