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Subject: Re: [PATCH] Fix left over EFI cache mapping problems
From: "Huang, Ying" <ying.huang@intel.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: Andi Kleen <andi@firstfloor.org>, torvalds@osdl.org, tglx@linutronix.de,
       linux-kernel@vger.kernel.org
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Date: Mon, 18 Feb 2008 09:53:56 +0800
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On Fri, 2008-02-15 at 08:08 +0100, Ingo Molnar wrote:
> * Huang, Ying <ying.huang@intel.com> wrote:
> 
> > On Thu, 2008-02-14 at 17:12 +0100, Ingo Molnar 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.
> > > 
> > > Ying, if you agree with this fix could you please test and ACK it before 
> > > we push it to Linus? (this fix is also in the latest x86.git#mm)
> > 
> > I think the patch following may be better, because it is possible that 
> > the EFI_PAGE_SHIFT and PAGE_SHIFT are different.
> 
> right now, EFI page size is 4096:
> 
>   include/linux/efi.h:#define EFI_PAGE_SHIFT          12
> 
> i doubt we'll ever change PAGE_SIZE on x86 - ia64's variable lowlevel 
> pagesizes are not particularly useful IMO. I think we'll at most have 
> some generic kernel feature that allows a larger PAGE_CACHE_SHIFT - but 
> on the lowlevel MMU level we'll always stay at 4K.
> 
> and i doubt EFI_PAGE_SHIFT would want to (ever) go away from 12 either. 

After looking up in UEFI specification, I found that it is specified by
UEFI specification 2.1 (section 6.2, page 124) that the NumberOfPages
(num_pages) of EFI_MEMORY_DESCRIPTOR (efi_memory_desc_t) must be "Number
of 4KB pages in the memory region". So we need not worry about potential
EFI_PAGE_SHIFT changes.

Best Regards,
Huang Ying

> So perhaps, at least as far as arch/x86/kernel/efi*.c files go, it would 
> be cleaner to just replace EFI_PAGE_SHIFT with PAGE_SHIFT and 
> EFI_PAGE_SIZE with PAGE_SIZE?