Re: [PATCH 3/6] mmu: map small pages into big pages(s) by IOMMU if possible

[Terje Bergstrom <tbergstrom-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>]

On 04/17/2015 02:11 AM, Alexandre Courbot wrote:
> Tracking the PDE and PTE of each memory chunk can probably be avoided
> if you change your unmapping strategy. Currently you are going through
> the list of nvkm_vm_bp_list, but you know your PDE and PTE are always
> going to be adjacent, since a nvkm_vma represents a contiguous block
> in the GPU VA. So when unmapping, you can simply check for each PTE
> entry whether the IOMMU bit is set, and unmap from the IOMMU space
> after unmapping from the GPU VA space, in a loop similar to that of
> nvkm_vm_unmap_at().
>
> Then we only need priv. You are keeping the nvkm_mm_node of the IOMMU
> space into it, and you need it to free the IOMMU VA space. If only we
> could find another way to store it, we could get rid of the whole
> structure and associated list_head in nvkm_vma...
>
> I need to give it some more thoughts, and we will probably need to
> change a few things in base.c to make the hooks more flexible, so
> please give me some more time to think about it. :) I just wanted to
> share my thoughts so far in case this puts you on track.
The way you described it would make GPU MMU and IOMMU mappings 1:1. So 
when we map a buffer to GPU MMU, we always map page by page the buffer 
also to IOMMU. There are disadvantages here.

IOMMU addresses are global, and uses in the GPU caches. When a buffer is 
mapped multiple times to different graphics contexts, we want to avoid 
cache aliasing by mapping the buffer only once to IOMMU. We also want to 
unmap the buffer from IOMMU only once after all the instances of the 
buffer have been unmapped, or only when the buffer is actually freed to 
cache IOMMU mappings.

Doing IOMMU mapping for the whole buffer with dma_map_sg is also faster 
than mapping page by page, because you can do only one TLB invalidate in 
the end of the loop instead of after every page if you use dma_map_single.

All of these would talk for having IOMMU and GMMU mapping loops 
separate. This patch set does not implement both the advantages above, 
but your suggestion would take us further away from that than Vince's 
version.
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