linux-kernel - Re: [PATCH] iommu: Improve the performance for direct

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Date:   Tue, 24 Nov 2020 11:05:21 +0000
From:   Will Deacon <will@...nel.org>
To:     Yong Wu <yong.wu@...iatek.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Tomasz Figa <tfiga@...gle.com>,
        linux-mediatek@...ts.infradead.org, srv_heupstream@...iatek.com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, youlin.pei@...iatek.com,
        Nicolas Boichat <drinkcat@...omium.org>, anan.sun@...iatek.com,
        chao.hao@...iatek.com
Subject: Re: [PATCH] iommu: Improve the performance for direct_mapping

On Tue, Nov 24, 2020 at 05:24:44PM +0800, Yong Wu wrote:
> On Mon, 2020-11-23 at 12:32 +0000, Will Deacon wrote:
> > On Fri, Nov 20, 2020 at 05:06:28PM +0800, Yong Wu wrote:
> > > +				unmapped_sz = 0;
> > > +			}
> > > +			start += pg_size;
> > > +		}
> > > +		if (unmapped_sz) {
> > > +			ret = iommu_map(domain, start, start, unmapped_sz,
> > > +					entry->prot);
> > 
> > Can you avoid this hunk by changing your loop check to something like:
> > 
> > 	if (!phys_addr) {
> > 		map_size += pg_size;
> > 		if (addr + pg_size < end)
> > 			continue;
> > 	}
> 
> Thanks for your quick review. I have fixed and tested it. the patch is
> simple. I copy it here. Is this readable for you now?
> 
> 
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -737,6 +737,7 @@ static int
> iommu_create_device_direct_mappings(struct iommu_group *group,
>  	/* We need to consider overlapping regions for different devices */
>  	list_for_each_entry(entry, &mappings, list) {
>  		dma_addr_t start, end, addr;
> +		size_t map_size = 0;
>  
>  		if (domain->ops->apply_resv_region)
>  			domain->ops->apply_resv_region(dev, domain, entry);
> @@ -752,12 +753,21 @@ static int
> iommu_create_device_direct_mappings(struct iommu_group *group,
>  			phys_addr_t phys_addr;
>  
>  			phys_addr = iommu_iova_to_phys(domain, addr);
> -			if (phys_addr)
> -				continue;
> +			if (!phys_addr) {
> +				map_size += pg_size;
> +				if (addr + pg_size < end)
> +					continue;
> +				else

You don't need the 'else' here  ^^^

> +					addr += pg_size; /*Point to End */

addr = end ?

That said, maybe we could simplify this further by changing the loop bounds
to be:

	for (addr = start; addr <= end; addr += pg_size)

and checking:

	if (!phys_addr && addr != end) {
		map_size += pg_size;
		continue;
	}

does that work?

Will