linux-kernel - Re: [PATCH v1 00/17] Provide a new two step DMA mapping API

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Message-ID: <20241108150226.GM35848@ziepe.ca>
Date: Fri, 8 Nov 2024 11:02:26 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Christoph Hellwig <hch@....de>
Cc: Robin Murphy <robin.murphy@....com>, Leon Romanovsky <leon@...nel.org>,
	Jens Axboe <axboe@...nel.dk>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Sagi Grimberg <sagi@...mberg.me>,
	Keith Busch <kbusch@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Logan Gunthorpe <logang@...tatee.com>,
	Yishai Hadas <yishaih@...dia.com>,
	Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
	Kevin Tian <kevin.tian@...el.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
	linux-rdma@...r.kernel.org, iommu@...ts.linux.dev,
	linux-nvme@...ts.infradead.org, linux-pci@...r.kernel.org,
	kvm@...r.kernel.org, linux-mm@...ck.org, matthew.brost@...el.com,
	Thomas.Hellstrom@...ux.intel.com, brian.welty@...el.com,
	himal.prasad.ghimiray@...el.com, krishnaiah.bommu@...el.com,
	niranjana.vishwanathapura@...el.com
Subject: Re: [PATCH v1 00/17] Provide a new two step DMA mapping API

On Thu, Nov 07, 2024 at 02:50:25PM +0100, Christoph Hellwig wrote:
> On Thu, Nov 07, 2024 at 09:28:08AM -0400, Jason Gunthorpe wrote:
> > Once we are freed from scatterlist we can explore a design that would
> > pass the P2P routing information directly. For instance imagine
> > something like:
> > 
> >    dma_map_p2p(dev, phys, p2p_provider);
> > 
> > Then dma_map_page(dev, page) could be something like
> > 
> >    if (is_pci_p2pdma_page(page))
> >       dev_map_p2p(dev, page_to_phys(page), page->pgmap->p2p_provider)
> 
> One thing that this series does is to move the P2P mapping decisions out
> of the low-level dma mapping helpers and into the caller (again) for
> the non-sg callers and moves the special switch based bus mapping into
> a routine that can be called directly.
> 
> Take a look at blk_rq_dma_map_iter_start, which now literally uses
> dma_map_page for the no-iommu, no-switch P2P case.  It also is a good
> use case for the proposed dma_map_phys.

It is fully OK? Can't dma_map_page() trigger swiotlb? It must not do
that for P2P. How does it know the difference if it just gets a phys?

Jason