linux-kernel - Re: [RFC PATCH v1 00/18] Provide a new two step DMA API mapping API

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Message-ID: <20240711232917.GR14050@ziepe.ca>
Date: Thu, 11 Jul 2024 20:29:17 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Christoph Hellwig <hch@....de>
Cc: Leon Romanovsky <leon@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Keith Busch <kbusch@...nel.org>,
	"Zeng, Oak" <oak.zeng@...el.com>,
	Chaitanya Kulkarni <kch@...dia.com>,
	Sagi Grimberg <sagi@...mberg.me>,
	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>,
	linux-block@...r.kernel.org, linux-kernel@...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
Subject: Re: [RFC PATCH v1 00/18] Provide a new two step DMA API mapping API

On Wed, Jul 10, 2024 at 08:22:12AM +0200, Christoph Hellwig wrote:
> On Tue, Jul 09, 2024 at 04:03:20PM -0300, Jason Gunthorpe wrote:
> > > Except for the powerpc bypass IOMMU or not is a global decision,
> > > and the bypass is per I/O.  So I'm not sure what else you want there?
> > 
> > For P2P we know if the DMA will go through the IOMMU or not based on
> > the PCIe fabric path between the initiator (the one doing the DMA) and
> > the target (the one providing the MMIO memory).
> 
> Oh, yes.  So effectively you are asking if we can arbitrarily mix
> P2P sources in a single map request.  I think the only sane answer
> from the iommu/dma subsystem perspective is: hell no.

Well, today we can mix them and the dma_map_sg will sort it out. With
this new API we can't anymore.

So this little detail needs to be taken care of somehow as well, and I
didn't see it in this RFC.

> For the block layer just having one kind per BIO is fine right now,
> although I could see use cases where people would want to combine
> them.  We can probably defer that until it is needed, though.

Do you have an application in mind that would want multi-kind per BIO?

Jason