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:	Thu, 13 Nov 2014 08:51:59 -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 November 13, 2014 4:15:26 AM EST, Juergen Gross <jgross@...e.com> wrote:
>On 11/12/2014 11:18 PM, Konrad Rzeszutek Wilk wrote:
>> 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/
>
>No. It's not a matter of the caller, but of the p2m list entry.
>
>>> soon as the switch to a linear mapped p2m list has been done.
>>
>> But your inline function still calls get_phys_to_machine?
>
>Sure. The switch is done in the next patch. David asked me to split
>the patch doing the preparation by adding __pfn_tom_mfn() in an own
>patch.
>
>>
>>
>>>
>>> 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.
>
>I'll reword the comment.
>
>>
>>> + * - __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?
>
>The next patch in this series does.

Then this should be part of the next patch
>
>>
>>> + * - 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'.
>
>That's a matter of taste, I think. I can change it if nobody else
>objects.
>

That would need to be a separate patch if we wanted to go that route.

>
>Juergen


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