linux-kernel - Re: [RFC PATCH 1/7] swiotlb: Instead of tracking the end of the swiotlb region just calculate it

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Message-ID: <506DB121.3020700@intel.com>
Date:	Thu, 04 Oct 2012 08:54:09 -0700
From:	Alexander Duyck <alexander.h.duyck@...el.com>
To:	Konrad Rzeszutek Wilk <konrad@...nel.org>
CC:	konrad.wilk@...cle.com, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, rob@...dley.net, akpm@...ux-foundation.org,
	joerg.roedel@....com, bhelgaas@...gle.com, shuahkhan@...il.com,
	linux-kernel@...r.kernel.org, devel@...uxdriverproject.org,
	x86@...nel.org
Subject: Re: [RFC PATCH 1/7] swiotlb: Instead of tracking the end of the swiotlb
 region just calculate it

On 10/04/2012 06:01 AM, Konrad Rzeszutek Wilk wrote:
> On Wed, Oct 03, 2012 at 05:38:47PM -0700, Alexander Duyck wrote:
>> In the case of swiotlb we already have the start of the region and the number
>> of slabs that give us the region size.  Instead of having to call
>> virt_to_phys on two pointers we can just take advantage of the fact that the
>> region is linear and just compute the end based on the start plus the size.
> Why not take advantage of 'the fact that the region is linear' and just
> pre-compute the end in swiotlb_init_with_tbl?
>
> That way the logic in is_swiotlb_buffer is even simpler?
>

Using a pre-computed end point based on a virtual address is more
expensive in the x86_64 case.  The calls to __phys_addr require a
separate function call.  By just using the physical address of the start
and adding the offset I can avoid the second call and the compiler will
take advantage of the smaller function size.  The result is that
is_swiotlb_buffer will be inlined.

Thanks,

Alex
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