Re: [PATCH v10 01/17] iova: Export alloc_iova_fast() and free_iova_fast()

From: Robin Murphy
Date: Wed Aug 04 2021 - 11:43:58 EST

On 2021-08-04 06:02, Yongji Xie wrote:
On Tue, Aug 3, 2021 at 6:54 PM Robin Murphy <robin.murphy@xxxxxxx> wrote:

On 2021-08-03 09:54, Yongji Xie wrote:
On Tue, Aug 3, 2021 at 3:41 PM Jason Wang <jasowang@xxxxxxxxxx> wrote:


在 2021/7/29 下午3:34, Xie Yongji 写道:
Export alloc_iova_fast() and free_iova_fast() so that
some modules can use it to improve iova allocation efficiency.


It's better to explain why alloc_iova() is not sufficient here.


Fine.

What I fail to understand from the later patches is what the IOVA domain
actually represents. If the "device" is a userspace process then
logically the "IOVA" would be the userspace address, so presumably
somewhere you're having to translate between this arbitrary address
space and actual usable addresses - if you're worried about efficiency
surely it would be even better to not do that?


Yes, userspace daemon needs to translate the "IOVA" in a DMA
descriptor to the VA (from mmap(2)). But this actually doesn't affect
performance since it's an identical mapping in most cases.

I'm not familiar with the vhost_iotlb stuff, but it looks suspiciously like you're walking yet another tree to make those translations. Even if the buffer can be mapped all at once with a fixed offset such that each DMA mapping call doesn't need a lookup for each individual "IOVA" - that might be what's happening already, but it's a bit hard to follow just reading the patches in my mail client - vhost_iotlb_add_range() doesn't look like it's super-cheap to call, and you're serialising on a lock for that.

My main point, though, is that if you've already got something else keeping track of the actual addresses, then the way you're using an iova_domain appears to be something you could do with a trivial bitmap allocator. That's why I don't buy the efficiency argument. The main design points of the IOVA allocator are to manage large address spaces while trying to maximise spatial locality to minimise the underlying pagetable usage, and allocating with a flexible limit to support multiple devices with different addressing capabilities in the same address space. If none of those aspects are relevant to the use-case - which AFAICS appears to be true here - then as a general-purpose resource allocator it's rubbish and has an unreasonably massive memory overhead and there are many, many better choices.

FWIW I've recently started thinking about moving all the caching stuff out of iova_domain and into the iommu-dma layer since it's now a giant waste of space for all the other current IOVA users.

Presumably userspace doesn't have any concern about alignment and the
things we have to worry about for the DMA API in general, so it's pretty
much just allocating slots in a buffer, and there are far more effective
ways to do that than a full-blown address space manager.

Considering iova allocation efficiency, I think the iova allocator is
better here. In most cases, we don't even need to hold a spin lock
during iova allocation.

If you're going
to reuse any infrastructure I'd have expected it to be SWIOTLB rather
than the IOVA allocator. Because, y'know, you're *literally implementing
a software I/O TLB* ;)


But actually what we can reuse in SWIOTLB is the IOVA allocator.

Huh? Those are completely unrelated and orthogonal things - SWIOTLB does not use an external allocator (see find_slots()). By SWIOTLB I mean specifically the library itself, not dma-direct or any of the other users built around it. The functionality for managing slots in a buffer and bouncing data in and out can absolutely be reused - that's why users like the Xen and iommu-dma code *are* reusing it instead of open-coding their own versions.

And
the IOVA management in SWIOTLB is not what we want. For example,
SWIOTLB allocates and uses contiguous memory for bouncing, which is
not necessary in VDUSE case.

alloc_iova() allocates a contiguous (in IOVA address) region of space. In vduse_domain_map_page() you use it to allocate a contiguous region of space from your bounce buffer. Can you clarify how that is fundamentally different from allocating a contiguous region of space from a bounce buffer? Nobody's saying the underlying implementation details of where the buffer itself comes from can't be tweaked.

And VDUSE needs coherent mapping which is
not supported by the SWIOTLB. Besides, the SWIOTLB works in singleton
mode (designed for platform IOMMU) , but VDUSE is based on on-chip
IOMMU (supports multiple instances).
That's not entirely true - the IOMMU bounce buffering scheme introduced in intel-iommu and now moved into the iommu-dma layer was already a step towards something conceptually similar. It does still rely on stealing the underlying pages from the global SWIOTLB pool at the moment, but the bouncing is effectively done in a per-IOMMU-domain context.

The next step is currently queued in linux-next, wherein we can now have individual per-device SWIOTLB pools. In fact at that point I think you might actually be able to do your thing without implementing any special DMA ops at all - you'd need to set up a pool for your "device" with force_bounce set, then when you mmap() that to userspace, set up dev->dma_range_map to describe an offset from the physical address of the buffer to the userspace address, and I think dma-direct would be tricked into doing the right thing. It's a bit wacky, but it could stand to save a hell of a lot of bother.

Finally, enhancing SWIOTLB to cope with virtually-mapped buffers that don't have to be physically contiguous is a future improvement which I think could benefit various use-cases - indeed it's possibly already on the table for IOMMU bounce pages - so would probably be welcome in general.

> So I still prefer to reuse the
> IOVA allocator to implement a MMU-based software IOTLB.

If you're dead set on open-coding all the bounce-buffering machinery, then I'd honestly recommend open-coding a more suitable buffer allocator as well ;)

Thanks,
Robin.