linux-kernel - Re: [PATCH V3 6/8] xen: Hide get_phys_to_machine() to be able to tune common path

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Date:	Wed, 12 Nov 2014 17:18:13 -0500
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Juergen Gross <jgross@...e.com>
Cc:	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	david.vrabel@...rix.com, boris.ostrovsky@...cle.com,
	x86@...nel.org, tglx@...utronix.de, mingo@...hat.com, hpa@...or.com
Subject: Re: [PATCH V3 6/8] xen: Hide get_phys_to_machine() to be able to
 tune common path

On Tue, Nov 11, 2014 at 06:43:44AM +0100, Juergen Gross wrote:
> Today get_phys_to_machine() is always called when the mfn for a pfn
> is to be obtained. Add a wrapper __pfn_to_mfn() as inline function
> to be able to avoid calling get_phys_to_machine() when possible as

s/when/where/
> soon as the switch to a linear mapped p2m list has been done.

But your inline function still calls get_phys_to_machine?


> 
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
>  arch/x86/include/asm/xen/page.h | 27 +++++++++++++++++++++------
>  arch/x86/xen/mmu.c              |  2 +-
>  arch/x86/xen/p2m.c              |  6 +++---
>  3 files changed, 25 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/include/asm/xen/page.h b/arch/x86/include/asm/xen/page.h
> index 28fa795..07d8a7b 100644
> --- a/arch/x86/include/asm/xen/page.h
> +++ b/arch/x86/include/asm/xen/page.h
> @@ -59,6 +59,22 @@ extern int clear_foreign_p2m_mapping(struct gnttab_unmap_grant_ref *unmap_ops,
>  				     struct page **pages, unsigned int count);
>  extern unsigned long m2p_find_override_pfn(unsigned long mfn, unsigned long pfn);
>  
> +/*
> + * When to use pfn_to_mfn(), __pfn_to_mfn() or get_phys_to_machine():
> + * - pfn_to_mfn() returns either INVALID_P2M_ENTRY or the mfn. In case of an
> + *   identity entry the identity indicator will be cleared.

Why don't you say : In case of identity PFN the same PFN is returned.

But you did miss that also the FOREIGN_FRAME_BIT is cleared.

> + * - __pfn_to_mfn() returns the found entry of the p2m table. A possibly set

s/of the/in the/
> + *   identity indicator will be still set. __pfn_to_mfn() is encapsulating
.. be still set if the PFN is an identity one.
> + *   get_phys_to_machine() and might skip that function if possible to speed
> + *   up the common path.

How is is skipping that function? The patch below does no such thing?

> + * - get_phys_to_machine() is basically the same as __pfn_to_mfn(), but
> + *   without any short cuts for the common fast path.

Right. Perhpas we should call it 'slow_p2m' instead of the 'get_phys_to_machine'.

> + */
> +static inline unsigned long __pfn_to_mfn(unsigned long pfn)
> +{
> +	return get_phys_to_machine(pfn);
> +}
> +
--
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