linux-kernel - Re: [PATCH 2/7] iommu/iova: cut down judgement times

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Date:   Thu, 23 Mar 2017 12:11:56 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Zhen Lei <thunder.leizhen@...wei.com>,
        Joerg Roedel <joro@...tes.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Sudeep Dutt <sudeep.dutt@...el.com>,
        Ashutosh Dixit <ashutosh.dixit@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Cc:     Zefan Li <lizefan@...wei.com>, Xinwei Hu <huxinwei@...wei.com>,
        Tianhong Ding <dingtianhong@...wei.com>,
        Hanjun Guo <guohanjun@...wei.com>
Subject: Re: [PATCH 2/7] iommu/iova: cut down judgement times

On 22/03/17 06:27, Zhen Lei wrote:
> Below judgement can only be satisfied at the last time, which produced 2N
> judgements(suppose N times failed, 0 or 1 time successed) in vain.
> 
> if ((pfn >= iova->pfn_lo) && (pfn <= iova->pfn_hi)) {
> 	return iova;
> }

For me, GCC (6.2.1 AArch64) seems to do a pretty good job of this
function already, so this change only saves two instructions in total
(pfn is compared against pfn_lo only once instead of twice), which I
wouldn't expect to see a noticeable performance effect from.

Given the improvement in readability, though, I don't even care about
any codegen differences :)

Reviewed-by: Robin Murphy <robin.murphy@....com>

> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> ---
>  drivers/iommu/iova.c | 9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> index 8ba8b496..1c49969 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -312,15 +312,12 @@ private_find_iova(struct iova_domain *iovad, unsigned long pfn)
>  	while (node) {
>  		struct iova *iova = rb_entry(node, struct iova, node);
>  
> -		/* If pfn falls within iova's range, return iova */
> -		if ((pfn >= iova->pfn_lo) && (pfn <= iova->pfn_hi)) {
> -			return iova;
> -		}
> -
>  		if (pfn < iova->pfn_lo)
>  			node = node->rb_left;
> -		else if (pfn > iova->pfn_lo)
> +		else if (pfn > iova->pfn_hi)
>  			node = node->rb_right;
> +		else
> +			return iova;	/* pfn falls within iova's range */
>  	}
>  
>  	return NULL;
>