linux-kernel - Re: [PATCH] Fix left over EFI cache mapping problems

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Message-ID: <20080214214241.GB19473@one.firstfloor.org>
Date:	Thu, 14 Feb 2008 22:42:41 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andi Kleen <andi@...stfloor.org>, torvalds@...l.org,
	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	ying.huang@...el.com
Subject: Re: [PATCH] Fix left over EFI cache mapping problems

On Thu, Feb 14, 2008 at 07:38:19PM +0100, Ingo Molnar wrote:
> 
> * Andi Kleen <andi@...stfloor.org> wrote:
> 
> > > this is indeed a bug (we change the attributes for a larger area 
> > > than needed), but your fix is unclean. Find below a cleaner 
> > > solution.
> > 
> > You're still ignoring the other problem of set_memory_uc() not 
> > handling fixmap and ioremap correctly. [...]
> 
> No, we did not ignore it, and yes, you are wrong.
> 
> One thing that you miss is that the 64-bit EFI runtime has to be marked 
> uncacheable only if it the EFI image attribute signals an uncacheable 
> area:
> 
>                 if (!(md->attribute & EFI_MEMORY_WB))
>                         set_memory_uc(md->virt_addr, md->num_pages);
> 
> and Linux EFI does not support device EFI runtimes. So your observation, 

Sorry I didn't get that (you were a bit terse). 

You're saying the EFI BIOSes will never set that flag ?

I'm reading page 123+ of UEFI 2.1 which describes GetMemoryMap() 
and these flags and I see nothing to that effect. I admit I didn't
read the full EFI bible so far so there are certainly EFI
aspects I don't understand.

Can you please clarify why EFI would not set that flag on Linux? 
Can you refer me to the parts of the spec that describe that? 

> Also note that 64-bit EFI runtime support (the ability to execute EFI 
> code) is completely new - it got introduced 14 days ago. We only use 
> fixmaps on 64-bit EFI.

On 32bit it is wrong too I think at least on non default __PAGE_OFFSET
splits.

> 
> 32-bit EFI is more common (but still not very common, compared to other 
> x86 platforms) and that is totally unaffected by secondary aliases. 

Of course it is affected. set_memory_uc() will not fix up the 
direct mapping in this case either. Given the overlap of PCI hole
to direct mapping cases there are more seldom, but certainly
exist (e.g. consider 1:3 split and a 2GB PCI hole) 

And while given that's a relatively obscure case it's a valid regression.

-Andi

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