linux-kernel - Re: [PATCH v1 13/16] mm/hmm: properly take MMIO path

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Message-ID: <20250807131418.GJ184255@nvidia.com>
Date: Thu, 7 Aug 2025 10:14:18 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Leon Romanovsky <leon@...nel.org>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>,
	Leon Romanovsky <leonro@...dia.com>,
	Abdiel Janulgue <abdiel.janulgue@...il.com>,
	Alexander Potapenko <glider@...gle.com>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@....de>, Danilo Krummrich <dakr@...nel.org>,
	iommu@...ts.linux.dev, Jason Wang <jasowang@...hat.com>,
	Jens Axboe <axboe@...nel.dk>, Joerg Roedel <joro@...tes.org>,
	Jonathan Corbet <corbet@....net>, Juergen Gross <jgross@...e.com>,
	kasan-dev@...glegroups.com, Keith Busch <kbusch@...nel.org>,
	linux-block@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-nvme@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
	linux-trace-kernel@...r.kernel.org,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Robin Murphy <robin.murphy@....com>, rust-for-linux@...r.kernel.org,
	Sagi Grimberg <sagi@...mberg.me>,
	Stefano Stabellini <sstabellini@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	virtualization@...ts.linux.dev, Will Deacon <will@...nel.org>,
	xen-devel@...ts.xenproject.org
Subject: Re: [PATCH v1 13/16] mm/hmm: properly take MMIO path

On Mon, Aug 04, 2025 at 03:42:47PM +0300, Leon Romanovsky wrote:
> From: Leon Romanovsky <leonro@...dia.com>
> 
> 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 <leonro@...dia.com>
> ---
>  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 <jgg@...dia.com>

Jason