linux-kernel - Re: [PATCH v1 1/4] mm/hmm: HMM API for P2P DMA to device zone pages

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Message-ID: <20241017134644.GA948948@ziepe.ca>
Date: Thu, 17 Oct 2024 10:46:44 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Christoph Hellwig <hch@...radead.org>
Cc: Yonatan Maman <ymaman@...dia.com>, nouveau@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
	linux-mm@...ck.org, herbst@...hat.com, lyude@...hat.com,
	dakr@...hat.com, airlied@...il.com, simona@...ll.ch,
	leon@...nel.org, jglisse@...hat.com, akpm@...ux-foundation.org,
	dri-devel@...ts.freedesktop.org, apopple@...dia.com,
	bskeggs@...dia.com, Gal Shalom <GalShalom@...dia.com>
Subject: Re: [PATCH v1 1/4] mm/hmm: HMM API for P2P DMA to device zone pages

On Thu, Oct 17, 2024 at 06:12:55AM -0700, Christoph Hellwig wrote:
> On Thu, Oct 17, 2024 at 10:05:39AM -0300, Jason Gunthorpe wrote:
> > Broadly I think whatever flow NVMe uses for P2P will apply to ODP as
> > well.
> 
> ODP is a lot simpler than NVMe for P2P actually :(

What is your thinking there? I'm looking at the latest patches and I
would expect dma_iova_init() to accept a phys so it can call
pci_p2pdma_map_type() once for the whole transaction. It is a slow
operation.

Based on the result of pci_p2pdma_map_type() it would have to take one
of three paths: direct, iommu, or acs/switch.

It feels like dma_map_page() should become a new function that takes
in the state and then it can do direct or acs based on the type held
in the state.

ODP would have to refresh the state for each page, but could follow
the same code structure.

Jason