Subject: Re: [PATCH v1 13/16] mm/hmm: properly take MMIO path

From: Jason Gunthorpe <[email-masked]>
Date: Thu, 7 Aug 2025 10:14:18 -0300

On Mon, Aug 04, 2025 at 03:42:47PM +0300, Leon Romanovsky wrote:
> From: Leon Romanovsky <[email-masked]>
> 
> In case peer-to-peer transaction traverses through host bridge,
> the IOMMU needs to have IOMMU_MMIO flag, together with skip of
> CPU sync.
> 
> The latter was handled by provided DMA_ATTR_SKIP_CPU_SYNC flag,
> but IOMMU flag was missed, due to assumption that such memory
> can be treated as regular one.
> 
> Reuse newly introduced DMA attribute to properly take MMIO path.
> 
> Signed-off-by: Leon Romanovsky <[email-masked]>
> ---
>  mm/hmm.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 015ab243f0813..6556c0e074ba8 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -746,7 +746,7 @@ dma_addr_t hmm_dma_map_pfn(struct device *dev, struct hmm_dma_map *map,
>  	case PCI_P2PDMA_MAP_NONE:
>  		break;
>  	case PCI_P2PDMA_MAP_THRU_HOST_BRIDGE:
> -		attrs |= DMA_ATTR_SKIP_CPU_SYNC;
> +		attrs |= DMA_ATTR_MMIO;
>  		pfns[idx] |= HMM_PFN_P2PDMA;
>  		break;

Yeah, this is a lot cleaner

Reviewed-by: Jason Gunthorpe <[email-masked]>

Jason