On 2023-09-25 04:59, Kelly Devilliv wrote:
> 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?
Not really. Other use-cases for dma_map_resource() include DMA offload
engines accessing FIFO registers, where allowing reordering,
write-gathering, etc. would be a terrible idea. Thus it needs to assume
a "safe" MMIO memory type, which on Arm means Device-nGnRE.
However, the "proper" PCI peer-to-peer support under CONFIG_PCI_P2PDMA
ended up moving away from the dma_map_resource() approach anyway, and
allows this kind of device memory to be treated more like regular memory
(via ZONE_DEVICE) rather than arbitrary MMIO resources, so your best bet
would be to get the GPU driver converted over to using that.
Thanks,
Robin.
>
> Thank you!
>
> Platform info:
> Linux kernel version: 5.10
> PCIE GEN4 x16
>
> Sincerely,
> Kelly
>