From: "Yang, Shunyong" <shunyong.yang@hxt-semitech.com>
To: "robin.murphy@arm.com" <robin.murphy@arm.com>,
"thunder.leizhen@huawei.com" <thunder.leizhen@huawei.com>,
"will.deacon@arm.com" <will.deacon@arm.com>
Cc: "linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>,
"joro@8bytes.org" <joro@8bytes.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"iommu@lists.linux-foundation.org"
<iommu@lists.linux-foundation.org>,
"jean-philippe.brucker@arm.com" <jean-philippe.brucker@arm.com>
Subject: Re: [PATCH v3 4/6] iommu/io-pgtable-arm: add support for non-strict mode
Date: Wed, 15 Aug 2018 01:43:37 +0000 [thread overview]
Message-ID: <7a2dedda98aa9e677eb7f85b6b55e34e0128d2d9.camel@hxt-semitech.com> (raw)
In-Reply-To: <feabf4a3-db1e-a22b-daab-fad831b473f9@arm.com>
Hi, Robin,
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?
Thanks.
Shunyong.
>
> Robin.
next prev parent reply other threads:[~2018-08-15 1:43 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-12 6:18 [PATCH v3 0/6] add non-strict mode support for arm-smmu-v3 Zhen Lei
2018-07-12 6:18 ` [PATCH v3 1/6] iommu/arm-smmu-v3: fix the implementation of flush_iotlb_all hook Zhen Lei
2018-07-12 6:18 ` [PATCH v3 2/6] iommu/dma: add support for non-strict mode Zhen Lei
2018-07-24 22:01 ` Robin Murphy
2018-07-26 4:15 ` Leizhen (ThunderTown)
2018-07-12 6:18 ` [PATCH v3 3/6] iommu/amd: use default branch to deal with all non-supported capabilities Zhen Lei
2018-07-12 6:18 ` [PATCH v3 4/6] iommu/io-pgtable-arm: add support for non-strict mode Zhen Lei
2018-07-24 22:25 ` Robin Murphy
2018-07-26 7:20 ` Leizhen (ThunderTown)
2018-07-26 14:35 ` Robin Murphy
2018-08-06 1:32 ` Yang, Shunyong
2018-08-14 8:33 ` Leizhen (ThunderTown)
2018-08-14 8:35 ` Will Deacon
2018-08-14 10:02 ` Robin Murphy
2018-08-15 1:43 ` Yang, Shunyong [this message]
2018-08-15 7:33 ` Will Deacon
2018-08-15 7:35 ` Will Deacon
2018-08-16 0:43 ` Yang, Shunyong
2018-07-12 6:18 ` [PATCH v3 5/6] iommu/arm-smmu-v3: " Zhen Lei
2018-07-12 6:18 ` [PATCH v3 6/6] iommu/arm-smmu-v3: add bootup option "iommu_strict_mode" Zhen Lei
2018-07-24 22:46 ` Robin Murphy
2018-07-26 7:41 ` Leizhen (ThunderTown)
2018-07-24 21:51 ` [PATCH v3 0/6] add non-strict mode support for arm-smmu-v3 Robin Murphy
2018-07-26 3:44 ` Leizhen (ThunderTown)
2018-07-26 14:16 ` Robin Murphy
2018-07-27 2:49 ` Leizhen (ThunderTown)
2018-07-27 9:37 ` Will Deacon
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