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!
Linux kernel version: 5.10
PCIE GEN4 x16