netdev - Re: [LSF/MM/BPF proposal]: Physr discussion

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Date:   Thu, 26 Jan 2023 09:45:21 +0800
From:   Zhu Yanjun <yanjun.zhu@...ux.dev>
To:     Jason Gunthorpe <jgg@...dia.com>, lsf-pc@...ts.linuxfoundation.org,
        linux-mm@...ck.org, iommu@...ts.linux.dev,
        linux-rdma@...r.kernel.org
Cc:     Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@....de>,
        Joao Martins <joao.m.martins@...cle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Ming Lei <ming.lei@...hat.com>, linux-block@...r.kernel.org,
        netdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        nvdimm@...ts.linux.dev
Subject: Re: [LSF/MM/BPF proposal]: Physr discussion

在 2023/1/21 23:03, Jason Gunthorpe 写道:
> I would like to have a session at LSF to talk about Matthew's
> physr discussion starter:
> 
>   https://lore.kernel.org/linux-mm/YdyKWeU0HTv8m7wD@casper.infradead.org/
> 
> I have become interested in this with some immediacy because of
> IOMMUFD and this other discussion with Christoph:
> 
>   https://lore.kernel.org/kvm/4-v2-472615b3877e+28f7-vfio_dma_buf_jgg@nvidia.com/

I read through the above patches. I am interested in the dma-buf.

Zhu Yanjun

>      
> Which results in, more or less, we have no way to do P2P DMA
> operations without struct page - and from the RDMA side solving this
> well at the DMA API means advancing at least some part of the physr
> idea.
> 
> So - my objective is to enable to DMA API to "DMA map" something that
> is not a scatterlist, may or may not contain struct pages, but can
> still contain P2P DMA data. From there I would move RDMA MR's to use
> this new API, modify DMABUF to export it, complete the above VFIO
> series, and finally, use all of this to add back P2P support to VFIO
> when working with IOMMUFD by allowing IOMMUFD to obtain a safe
> reference to the VFIO memory using DMABUF. From there we'd want to see
> pin_user_pages optimized, and that also will need some discussion how
> best to structure it.
> 
> I also have several ideas on how something like physr can optimize the
> iommu driver ops when working with dma-iommu.c and IOMMUFD.
> 
> I've been working on an implementation and hope to have something
> draft to show on the lists in a few weeks. It is pretty clear there
> are several interesting decisions to make that I think will benefit
> from a live discussion.
> 
> Providing a kernel-wide alternative to scatterlist is something that
> has general interest across all the driver subsystems. I've started to
> view the general problem rather like xarray where the main focus is to
> create the appropriate abstraction and then go about transforming
> users to take advatange of the cleaner abstraction. scatterlist
> suffers here because it has an incredibly leaky API, a huge number of
> (often sketchy driver) users, and has historically been very difficult
> to improve.
> 
> The session would quickly go over the current state of whatever the
> mailing list discussion evolves into and an open discussion around the
> different ideas.
> 
> Thanks,
> Jason
>