Messages in this thread | | | Subject | Re: [PATCH v2 3/8] iommu/dma: Disable get_sgtable for granule > PAGE_SIZE | From | Robin Murphy <> | Date | Fri, 3 Sep 2021 16:45:41 +0100 |
| |
On 2021-09-03 16:16, Sven Peter wrote: > > > On Thu, Sep 2, 2021, at 21:42, Robin Murphy wrote: >> On 2021-09-02 19:19, Sven Peter wrote: >>> >>> >>> On Wed, Sep 1, 2021, at 23:10, Alyssa Rosenzweig wrote: >>>>> My biggest issue is that I do not understand how this function is supposed >>>>> to be used correctly. It would work fine as-is if it only ever gets passed buffers >>>>> allocated by the coherent API but there's not way to check or guarantee that. >>>>> There may also be callers making assumptions that no longer hold when >>>>> iovad->granule > PAGE_SIZE. >>>>> >>>>> Regarding your case: I'm not convinced the function is meant to be used there. >>>>> If I understand it correctly, your code first allocates memory with dma_alloc_coherent >>>>> (which possibly creates a sgt internally and then maps it with iommu_map_sg), >>>>> then coerces that back into a sgt with dma_get_sgtable, and then maps that sgt to >>>>> another iommu domain with dma_map_sg while assuming that the result will be contiguous >>>>> in IOVA space. It'll work out because dma_alloc_coherent is the very thing >>>>> meant to allocate pages that can be mapped into kernel and device VA space >>>>> as a single contiguous block and because both of your IOMMUs are different >>>>> instances of the same HW block. Anything allocated by dma_alloc_coherent for the >>>>> first IOMMU will have the right shape that will allow it to be mapped as >>>>> a single contiguous block for the second IOMMU. >>>>> >>>>> What could be done in your case is to instead use the IOMMU API, >>>>> allocate the pages yourself (while ensuring the sgt your create is made up >>>>> of blocks with size and physaddr aligned to max(domain_a->granule, domain_b->granule)) >>>>> and then just use iommu_map_sg for both domains which actually comes with the >>>>> guarantee that the result will be a single contiguous block in IOVA space and >>>>> doesn't required the sgt roundtrip. >>>> >>>> In principle I agree. I am getting the sense this function can't be used >>>> correctly in general, and yet is the function that's meant to be used. >>>> If my interpretation of prior LKML discussion holds, the problems are >>>> far deeper than my code or indeed page size problems... >>> >>> Right, which makes reasoning about this function and its behavior if the >>> IOMMU pages size is unexpected very hard for me. I'm not opposed to just >>> keeping this function as-is when there's a mismatch between PAGE_SIZE and >>> the IOMMU page size (and it will probably work that way) but I'd like to >>> be sure that won't introduce unexpected behavior. >>> >>>> >>>> If the right way to handle this is with the IOMMU and IOVA APIs, I really wish >>>> that dance were wrapped up in a safe helper function instead of open >>>> coding it in every driver that does cross device sharing. >>>> >>>> We might even call that helper... hmm... dma_map_sg.... *ducks* >>>> >>> >>> There might be another way to do this correctly. I'm likely just a little >>> bit biased because I've spent the past weeks wrapping my head around the >>> IOMMU and DMA APIs and when all you have is a hammer everything looks like >>> a nail. >>> >>> But dma_map_sg operates at the DMA API level and at that point the dma-ops >>> for two different devices could be vastly different. >>> In the worst case one of them could be behind an IOMMU that can easily map >>> non-contiguous pages while the other one is directly connected to the bus and >>> can't even access >4G pages without swiotlb support. >>> It's really only possible to guarantee that it will map N buffers to <= N >>> DMA-addressable buffers (possibly by using an IOMMU or swiotlb internally) at >>> that point. >>> >>> On the IOMMU API level you have much more information available about the actual >>> hardware and can prepare the buffers in a way that makes both devices happy. >>> That's why iommu_map_sgtable combined with iovad->granule aligned sgt entries >>> can actually guarantee to map the entire list to a single contiguous IOVA block. >> >> Essentially there are two reasonable options, and doing pretend dma-buf >> export/import between two devices effectively owned by the same driver >> is neither of them. Handily, DRM happens to be exactly where all the >> precedent is, too; unsurprisingly this is not a new concern. >> >> One is to go full IOMMU API, like rockchip or tegra, attaching the >> relevant devices to your own unmanaged domain(s) and mapping pages >> exactly where you choose. You still make dma_map/dma_unmap calls for the >> sake of cache maintenance and other housekeeping on the underlying >> memory, but you ignore the provided DMA addresses in favour of your own >> IOVAs when it comes to programming the devices. >> >> The lazier option if you can rely on all relevant devices having equal >> DMA and IOMMU capabilities is to follow exynos, and herd the devices >> into a common default domain. Instead of allocating you own domain, you >> grab the current domain for one device (which will be its default >> domain) and manually attach the other devices to that. Then you forget >> all about IOMMUs but make sure to do all your regular DMA API calls >> using that first device, and the DMA addresses which come back should be >> magically valid for the other devices too. It was a bit of a cheeky hack >> TBH, but I'd still much prefer more of that over any usage of >> get_sgtable outside of actual dma-buf... >> >> Note that where multiple IOMMU instances are involved, the latter >> approach does depend on the IOMMU driver being able to support sharing a >> single domain across them; I think that might sort-of-work for DART >> already, but may need a little more attention. > > It'll work for two streams inside the same DART but needs some > attention for streams from two separate DARTs. > > Then there's also this amazing "feature" that the display controller DART > pagetable pointer register is read-only so that we have to reuse the memory > Apple configured for first level table. That needs some changes anyway > but might make adding multiple devices from different groups more complex.
OK, I was thinking the dual-DART accommodation is already at least some of the way there, but I guess it's still tied to a single device's cfg. One upside to generalising further might be that the dual-DART case stops being particularly special :)
Not being able to physically share pagetables shouldn't be too big a deal, just a bit more work to sync iommu_map/iommu_unmap calls across all the relevant instances for the given domain.
Robin.
| |