linux-kernel - Re: [PATCH 04/10] xen/mmu: For 1-1 mapping, automatically set _PAGE

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Date:	Tue, 21 Dec 2010 14:29:31 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	linux-kernel@...r.kernel.org, hpa@...or.com,
	Jan Beulich <JBeulich@...ell.com>,
	xen-devel@...ts.xensource.com,
	Konrad Rzeszutek Wilk <konrad@...nel.org>
Subject: Re: [PATCH 04/10] xen/mmu: For 1-1 mapping, automatically set _PAGE_IOMAP.

On 12/21/2010 01:37 PM, Konrad Rzeszutek Wilk wrote:
> This means that for PFNs (specifically: those in any E820 gaps
> or non-RAM E820 regions) that have 1-1 mapping we set the
> _PAGE_IOMAP flag.
>
> Later on we could remove the _PAGE_IOMAP code handling, but
> for right now lets keep this in to not introduce any bisection
> failures across this patchset.
>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
>  arch/x86/xen/mmu.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> index 4ba7e4e..bd02e7d 100644
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -832,6 +832,9 @@ static pteval_t pte_pfn_to_mfn(pteval_t val)
>  		pteval_t flags = val & PTE_FLAGS_MASK;
>  		unsigned long mfn = pfn_to_mfn(pfn);
>  
> +		if (mfn == pfn)
> +			flags |= _PAGE_IOMAP;

Why?  Does it really make sense to set _PAGE_IOMAP if they just happen
to be the same value?

    J
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