linux-kernel - Re: [PATCH v3 4/6] iommu/io-pgtable-arm: add support for non-strict mode

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Message-ID: <20180815073300.GA2100@arm.com>
Date:   Wed, 15 Aug 2018 08:33:01 +0100
From:   Will Deacon <will.deacon@....com>
To:     "Yang, Shunyong" <shunyong.yang@...-semitech.com>
Cc:     "robin.murphy@....com" <robin.murphy@....com>,
        "thunder.leizhen@...wei.com" <thunder.leizhen@...wei.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "joro@...tes.org" <joro@...tes.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "jean-philippe.brucker@....com" <jean-philippe.brucker@....com>
Subject: Re: [PATCH v3 4/6] iommu/io-pgtable-arm: add support for non-strict
 mode

On Wed, Aug 15, 2018 at 01:43:37AM +0000, Yang, Shunyong wrote:
> On Tue, 2018-08-14 at 11:02 +0100, Robin Murphy wrote:
> > On 14/08/18 09:35, Will Deacon wrote:
> > > 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?
> >
> > ...or GPU drivers, or anyone else managing their own IOMMU domain
> > directly. A sequence like this is perfectly legal:
> >
> >     iommu_map(domain, iova, paddr, SZ_8M, prot);
> >     ...
> >     iommu_unmap(domain, iova + SZ_1M * 5, SZ_1M * 3);
> >
> > where if iova and paddr happen to be suitably aligned, the map will
> > lay
> > down blocks, and the unmap will then have to split one of them into
> > pages to remove half of it. We don't tear our hair out maintaining
> > split_blk_unmap() for the fun of it :(
>
> Thank you for the GPU example. But for VFIO, I remember all memory will
> be   pinned in the early stage of emulator (such as qemu) start. So,
> the split will occur at which operation? Maybe virtio balloon inflate?

My memory is pretty hazy here, but I was fairly sure that VFIO didn't
always unmap() with the same granularity as it map()'d, at least for
the v1 interface. Either way, split_blk_unmap() was written because it was
necessary at the time, rather than just for fun!

Will
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