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

[Robin Murphy <robin.murphy@arm.com>]

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
>