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

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Message-ID: <20241017140507.GB948948@ziepe.ca>
Date: Thu, 17 Oct 2024 11:05:07 -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 06:49:30AM -0700, Christoph Hellwig wrote:
> On Thu, Oct 17, 2024 at 10:46:44AM -0300, Jason Gunthorpe wrote:
> > 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.
> 
> You can't do it for the whole transaction.  Here is my suggestion
> for ODP:
> 
> http://git.infradead.org/?p=users/hch/misc.git;a=shortlog;h=refs/heads/dma-split-wip

OK, this looks very promising. I sketched something similar to the
pci-p2pdma changes a while back too.

BTW this:

  iommu: generalize the batched sync after map interface

I am hoping to in a direction of adding a gather to the map, just like
unmap. So eventually instead of open coding iotlb_sync_map() you'd
flush the gather and it would do it.

> For NVMe I need to figure out a way to split bios on a per P2P
> type boundary as we don't have any space to record if something is a bus
> mapped address.

Yeah this came up before :\ Can't precompute the p2p type during bio
creation, splitting based on pgmap would be good enough.

Jason