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:

quoted

On 14/08/18 09:35, Will Deacon wrote:

quoted

On Tue, Aug 14, 2018 at 04:33:41PM +0800, Leizhen (ThunderTown)
wrote:

quoted

On 2018/8/6 9:32, Yang, Shunyong wrote:

quoted

On 2018/7/26 22:37, Robin Murphy wrote:

quoted

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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