dma_map_resource() has a bad performance in pcie peer to peer transactions when iommu enabled in Linux

[Kelly Devilliv]

Dear all,

I am working on an ARM-V8 server with two gpu cards on it. Recently, I need to test pcie peer to peer communication between the two gpu cards, but the throughput is only 4GB/s.

After I explored the gpu's kernel mode driver, I found it was using the dma_map_resource() API to map the peer device's MMIO space. The arm iommu driver then will hardcode a 'IOMMU_MMIO' prot in the later dma map:

       static dma_addr_t iommu_dma_map_resource(struct device *dev, phys_addr_t phys,
                                size_t size, enum dma_data_direction dir, unsigned long attrs)
        {
                return __iommu_dma_map(dev, phys, size,
                                        dma_info_to_prot(dir, false, attrs) | IOMMU_MMIO,
                                        dma_get_mask(dev));
        }

And that will finally set the 'ARM_LPAE_PTE_MEMATTR_DEV' attribute in PTE, which may have a negative impact on the performance of the pcie peer to peer transactions.

        /*
         * Note that this logic is structured to accommodate Mali LPAE
         * having stage-1-like attributes but stage-2-like permissions.
         */
        if (data->iop.fmt == ARM_64_LPAE_S2 ||
            data->iop.fmt == ARM_32_LPAE_S2) {
                if (prot & IOMMU_MMIO)
                        pte |= ARM_LPAE_PTE_MEMATTR_DEV;
                else if (prot & IOMMU_CACHE)
                        pte |= ARM_LPAE_PTE_MEMATTR_OIWB;
                else
                        pte |= ARM_LPAE_PTE_MEMATTR_NC;
        } else {
                if (prot & IOMMU_MMIO)
                        pte |= (ARM_LPAE_MAIR_ATTR_IDX_DEV
                                << ARM_LPAE_PTE_ATTRINDX_SHIFT);
                else if (prot & IOMMU_CACHE)
                        pte |= (ARM_LPAE_MAIR_ATTR_IDX_CACHE
                                << ARM_LPAE_PTE_ATTRINDX_SHIFT);
        }

I tried to remove the 'IOMMU_MMIO' prot in the dma_map_resource() API and re-compile the linux kernel, the throughput then can be up to 28GB/s.

Is there an elegant way to solve this issue without modifying the linux kernel? e.g., a substitution of dma_map_resource() API?

Thank you!

Platform info:
Linux kernel version: 5.10
PCIE GEN4 x16

Sincerely,
Kelly