From mboxrd@z Thu Jan  1 00:00:00 1970
From: will.deacon@arm.com (Will Deacon)
Date: Tue, 14 Aug 2018 09:35:01 +0100
Subject: [PATCH v3 4/6] iommu/io-pgtable-arm: add support for non-strict
 mode
In-Reply-To: <5B7293E5.7040702@huawei.com>
References: <1531376312-2192-1-git-send-email-thunder.leizhen@huawei.com>
 <1531376312-2192-5-git-send-email-thunder.leizhen@huawei.com>
 <89cc2201-99ab-3f3b-a2d1-1766515d4375@arm.com>
 <5B597628.2020103@huawei.com>
 <04239cfa-bcf2-a33a-e662-ebc75e66782b@arm.com>
 <1d24541340334954969c58980ef85444@HXTBJIDCEMVIW01.hxtcorp.net>
 <5B7293E5.7040702@huawei.com>
Message-ID: <20180814083500.GA28101@arm.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On Tue, Aug 14, 2018 at 04:33:41PM +0800, Leizhen (ThunderTown) wrote:
> On 2018/8/6 9:32, Yang, Shunyong wrote:
> > On 2018/7/26 22:37, Robin Murphy wrote:
> >> Because DMA code is not the only caller of iommu_map/unmap. It's 
> >> perfectly legal in the IOMMU API to partially unmap a previous mapping 
> >> such that a block entry needs to be split. The DMA API, however, is a 
> >> lot more constrined, and thus by construction the iommu-dma layer will 
> >> never generate a block-splitting iommu_unmap() except as a result of 
> >> illegal DMA API usage, and we obviously do not need to optimise for that 
> >> (you will get a warning about mismatched unmaps under dma-debug, but 
> >> it's a bit too expensive to police in the general case).
> >>
> > 
> > When I was reading the code around arm_lpae_split_blk_unmap(), I was
> > curious in which scenario a block will be split. Now with your comments
> > "Because DMA code is not the only caller of iommu_map/unmap", it seems
> > depending on the user.
> > 
> > Would you please explain this further? I mean besides DMA, which user
> > will use iommu_map/umap and how it split a block.
> 
> I also think that arm_lpae_split_blk_unmap() scenario is not exist, maybe
> we should remove it, and give a warning for this wrong usage.

Can't it happen with VFIO?

Will