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Subject: Re: [PATCH 7/8] xen/mmu: Introduce IDENTITY_FRAME_BIT
From: Ian Campbell <Ian.Campbell@eu.citrix.com>
To: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
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        Konrad Rzeszutek Wilk <konrad@kernel.org>,
        Stefano Stabellini <Stefano.Stabellini@eu.citrix.com>
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Date: Tue, 4 Jan 2011 16:26:15 +0000
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On Thu, 2010-12-30 at 19:48 +0000, Konrad Rzeszutek Wilk wrote:
> @@ -52,9 +53,12 @@ static inline unsigned long pfn_to_mfn(unsigned long pfn)
>  
>  	mfn = get_phys_to_machine(pfn);
>  
> -	if (mfn != INVALID_P2M_ENTRY)
> +	if (mfn != INVALID_P2M_ENTRY) {
>  		mfn &= ~FOREIGN_FRAME_BIT;
>  
> +		if (mfn & IDENTITY_FRAME_BIT)
> +			mfn &= ~IDENTITY_FRAME_BIT;
> +	}

I don't think the inner-if buys us anything here and the whole thing is
equivalent to:
	if (mfn != INVALID_P2M_ENTRY)
		mfn &= ~(FOREIGN_FRAME_BIT|IDENTITY_FRAME_BIT);

Not sure if the FOREIGN_FRAME_BIT|IDENTITY_FRAME_BIT construct gets
enough use to be worthy of a #define FRAME_TYPE_MASK etc.

>  	return mfn;
>  }
>  
> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> index d98bd43..d470435 100644
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -425,10 +425,10 @@ unsigned long get_phys_to_machine(unsigned long pfn)
>  	 * would be wrong.
>  	 */
>  	if (p2m_top[topidx] == p2m_mid_identity)
> -		return pfn;
> +		return pfn | IDENTITY_FRAME_BIT;

It's probably worth defining IDENTITY_FRAME(m) in the pattern of
FOREIGN_FRAME(m).

Ian.