linux-kernel - Re: [RFC RESEND 00/16] Split IOMMU DMA mapping operation to two steps

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Date: Sat, 9 Mar 2024 17:14:18 +0100
From: Christoph Hellwig <hch@....de>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Christoph Hellwig <hch@....de>, Leon Romanovsky <leon@...nel.org>,
	Robin Murphy <robin.murphy@....com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Chaitanya Kulkarni <chaitanyak@...dia.com>,
	Jonathan Corbet <corbet@....net>, Jens Axboe <axboe@...nel.dk>,
	Keith Busch <kbusch@...nel.org>, Sagi Grimberg <sagi@...mberg.me>,
	Yishai Hadas <yishaih@...dia.com>,
	Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
	Kevin Tian <kevin.tian@...el.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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,
	kvm@...r.kernel.org, linux-mm@...ck.org,
	Bart Van Assche <bvanassche@....org>,
	Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
	Amir Goldstein <amir73il@...il.com>,
	"josef@...icpanda.com" <josef@...icpanda.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	"daniel@...earbox.net" <daniel@...earbox.net>,
	Dan Williams <dan.j.williams@...el.com>,
	"jack@...e.com" <jack@...e.com>, Zhu Yanjun <zyjzyj2000@...il.com>
Subject: Re: [RFC RESEND 00/16] Split IOMMU DMA mapping operation to two
 steps

On Fri, Mar 08, 2024 at 04:23:42PM -0400, Jason Gunthorpe wrote:
> > The DMA API callers really need to know what is P2P or not for
> > various reasons.  And they should generally have that information
> > available, either from pin_user_pages that needs to special case
> > it or from the in-kernel I/O submitter that build it from P2P and
> > normal memory.
> 
> I think that is a BIO thing. RDMA just calls with FOLL_PCI_P2PDMA and
> shoves the resulting page list into in a scattertable. It never checks
> if any returned page is P2P - it has no reason to care. dma_map_sg()
> does all the work.

Right now it does, but that's not really a good interface.  If we have
a pin_user_pages variant that only pins until the next relevant P2P
boundary and tells you about we can significantly simplify the overall
interface.