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Message-ID: <20240309161418.GA27113@lst.de>
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>,
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Yishai Hadas <yishaih@...dia.com>,
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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,
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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.
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