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

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Date:   Wed, 25 Nov 2020 11:13:02 +0000
From:   Will Deacon <will@...nel.org>
To:     Yong Wu <yong.wu@...iatek.com>
Cc:     youlin.pei@...iatek.com, anan.sun@...iatek.com,
        Nicolas Boichat <drinkcat@...omium.org>,
        srv_heupstream@...iatek.com, chao.hao@...iatek.com,
        Joerg Roedel <joro@...tes.org>, linux-kernel@...r.kernel.org,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Tomasz Figa <tfiga@...gle.com>,
        iommu@...ts.linux-foundation.org,
        linux-mediatek@...ts.infradead.org,
        Matthias Brugger <matthias.bgg@...il.com>,
        Robin Murphy <robin.murphy@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] iommu: Improve the performance for direct_mapping

On Wed, Nov 25, 2020 at 07:03:34PM +0800, Yong Wu wrote:
> On Tue, 2020-11-24 at 11:05 +0000, Will Deacon wrote:
> > 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:
> > 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?
> 
> It works but I think we can not check iommu_iova_to_phys(domain, end).
> We should add a "if", like:
> 
> for (addr = start; addr <= end; addr += pg_size) {
> ...
> 	if (addr < end) {
> 		phys_addr = iommu_iova_to_phys(domain, addr);
> 		if (!phys_addr) {
> 			map_size += pg_size;
> 			continue;
> 		}
> 	}
> ...
> 

Oh yes, you're right.

> If you don't like this "if (addr < end)", then we have to add a "goto".
> like this:
> 
> 
> for (addr = start; addr <= end; addr += pg_size) {
>  	phys_addr_t phys_addr;
>  
> 	if (addr == end)
> 		goto map_last;
> 
> 	phys_addr = iommu_iova_to_phys(domain, addr);
> 	if (!phys_addr) {
> 		map_size += pg_size;
> 		continue;
> 	}
> 
> map_last:
> 	if (!map_size)
> 		continue;
> 	ret = iommu_map(domain, addr - map_size,
> 			addr - map_size, map_size, entry->prot);

I think it's cleared to invert this as you had before:

	if (map_size)
		ret = iommu_map(...);

> Which one is better?

The second one is easier to read. I'll stop making suggestions now, thanks.

Will