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Date: Fri, 16 Sep 2022 19:53:25 +0100 From: Robin Murphy <robin.murphy@....com> To: joro@...tes.org, will@...nel.org Cc: iommu@...ts.linux.dev, linux-kernel@...r.kernel.org, Linus Torvalds <torvalds@...ux-foundation.org> Subject: [PATCH] iommu: Optimise PCI SAC address trick Per the resasoning in commit 4bf7fda4dce2 ("iommu/dma: Add config for PCI SAC address trick") and its subsequent revert, this mechanism no longer serves it original purpose, but now only works around broken hardware/drivers in a way that is unfortunately too impactful to remove. This does not, however prevent us from solving the performance impact which that workaround has on large-scale systems that don't need it. That kicks in once the 32-bit IOVA space fills up and we keep unsuccessfully trying to allocate from it. However, if we get to that point then in fact it's already the endgame. The nature of the allocator is such that the first IOVA we give to a device after the 32-bit space runs out will be the highest possible address for that device, ever. If that works, then great, we know we can optimise for speed by always allocating from the full range. And if it doesn't, then the worst has already happened and any brokenness is now showing, so there's no point continuing to try to hide it. To that end, implement a flag to refine this into a per-device policy that can automatically get itself out of the way if and when it stops being useful. CC: Linus Torvalds <torvalds@...ux-foundation.org> Signed-off-by: Robin Murphy <robin.murphy@....com> --- drivers/iommu/dma-iommu.c | 5 ++++- drivers/iommu/iommu.c | 3 +++ include/linux/iommu.h | 2 ++ 3 files changed, 9 insertions(+), 1 deletion(-) diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c index 9297b741f5e8..1cebb16faa33 100644 --- a/drivers/iommu/dma-iommu.c +++ b/drivers/iommu/dma-iommu.c @@ -643,9 +643,12 @@ static dma_addr_t iommu_dma_alloc_iova(struct iommu_domain *domain, dma_limit = min(dma_limit, (u64)domain->geometry.aperture_end); /* Try to get PCI devices a SAC address */ - if (dma_limit > DMA_BIT_MASK(32) && !iommu_dma_forcedac && dev_is_pci(dev)) + if (dma_limit > DMA_BIT_MASK(32) && dev->iommu->pci_workaround) { iova = alloc_iova_fast(iovad, iova_len, DMA_BIT_MASK(32) >> shift, false); + if (!iova) + dev->iommu->pci_workaround = false; + } if (!iova) iova = alloc_iova_fast(iovad, iova_len, dma_limit >> shift, diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c index edc768bf8976..19d0a6daae73 100644 --- a/drivers/iommu/iommu.c +++ b/drivers/iommu/iommu.c @@ -323,6 +323,9 @@ static int __iommu_probe_device(struct device *dev, struct list_head *group_list iommu_device_link(iommu_dev, dev); + if (!iommu_dma_forcedac && dev_is_pci(dev)) + dev->iommu->pci_workaround = true; + return 0; out_release: diff --git a/include/linux/iommu.h b/include/linux/iommu.h index 79cb6eb560a8..0eb0f808109c 100644 --- a/include/linux/iommu.h +++ b/include/linux/iommu.h @@ -368,6 +368,7 @@ struct iommu_fault_param { * @fwspec: IOMMU fwspec data * @iommu_dev: IOMMU device this device is linked to * @priv: IOMMU Driver private data + * @pci_workaround: Limit DMA allocations to 32-bit IOVAs * * TODO: migrate other per device data pointers under iommu_dev_data, e.g. * struct iommu_group *iommu_group; @@ -379,6 +380,7 @@ struct dev_iommu { struct iommu_fwspec *fwspec; struct iommu_device *iommu_dev; void *priv; + bool pci_workaround; }; int iommu_device_register(struct iommu_device *iommu, -- 2.36.1.dirty
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