Message-ID: <506DB121.3020700@intel.com>
Date: Thu, 04 Oct 2012 08:54:09 -0700
From: Alexander Duyck <alexander.h.duyck@intel.com>
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To: Konrad Rzeszutek Wilk <konrad@kernel.org>
CC: konrad.wilk@oracle.com, tglx@linutronix.de, mingo@redhat.com,
        hpa@zytor.com, rob@landley.net, akpm@linux-foundation.org,
        joerg.roedel@amd.com, bhelgaas@google.com, shuahkhan@gmail.com,
        linux-kernel@vger.kernel.org, devel@linuxdriverproject.org,
        x86@kernel.org
Subject: Re: [RFC PATCH 1/7] swiotlb: Instead of tracking the end of the swiotlb
 region just calculate it
References: <20121004002113.5016.66913.stgit@gitlad.jf.intel.com> <20121004003847.5016.50978.stgit@gitlad.jf.intel.com> <20121004130107.GB9158@phenom.dumpdata.com>
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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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