Re: [RFC PATCH v4 0/2] RDMA/rxe: Add dma-buf support

From: Shunsuke Mie <mie@igel.co.jp>
To: "Christian König" <christian.koenig@amd.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>, Zhu Yanjun <zyjzyj2000@gmail.com>,
	Alex Deucher <alexander.deucher@amd.com>,
	Daniel Vetter <daniel.vetter@ffwll.ch>,
	Doug Ledford <dledford@redhat.com>,
	Jianxin Xiong <jianxin.xiong@intel.com>,
	Leon Romanovsky <leon@kernel.org>,
	Maor Gottlieb <maorg@nvidia.com>,
	Sean Hefty <sean.hefty@intel.com>,
	Sumit Semwal <sumit.semwal@linaro.org>,
	dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org,
	linux-media@vger.kernel.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-rdma <linux-rdma@vger.kernel.org>,
	Damian Hobson-Garcia <dhobsong@igel.co.jp>,
	Takanari Hayama <taki@igel.co.jp>,
	Tomohito Esaki <etom@igel.co.jp>
Subject: Re: [RFC PATCH v4 0/2] RDMA/rxe: Add dma-buf support
Date: Mon, 13 Dec 2021 20:18:57 +0900	[thread overview]
Message-ID: <CANXvt5rzmEnF3Gph4U6NT-XzJhV6zqyay1g7dHkTgH=Aqc6Geg@mail.gmail.com> (raw)
In-Reply-To: <d25b2895-63b6-158d-ff73-f05e437e0f91@amd.com>

2021年12月10日(金) 22:29 Christian König <christian.koenig@amd.com>:
>
> Am 10.12.21 um 14:26 schrieb Jason Gunthorpe:
> > On Fri, Dec 10, 2021 at 01:47:37PM +0100, Christian König wrote:
> >> Am 10.12.21 um 13:42 schrieb Jason Gunthorpe:
> >>> On Fri, Dec 10, 2021 at 08:29:24PM +0900, Shunsuke Mie wrote:
> >>>> Hi Jason,
> >>>> Thank you for replying.
> >>>>
> >>>> 2021年12月8日(水) 2:14 Jason Gunthorpe <jgg@ziepe.ca>:
> >>>>> On Fri, Dec 03, 2021 at 12:51:44PM +0900, Shunsuke Mie wrote:
> >>>>>> Hi maintainers,
> >>>>>>
> >>>>>> Could you please review this patch series?
> >>>>> Why is it RFC?
> >>>>>
> >>>>> I'm confused why this is useful?
> >>>>>
> >>>>> This can't do copy from MMIO memory, so it shouldn't be compatible
> >>>>> with things like Gaudi - does something prevent this?
> >>>> I think if an export of the dma-buf supports vmap, CPU is able to access the
> >>>> mmio memory.
> >>>>
> >>>> Is it wrong? If this is wrong, there is no advantages this changes..
> >>> I don't know what the dmabuf folks did, but yes, it is wrong.
> >>>
> >>> IOMEM must be touched using only special accessors, some platforms
> >>> crash if you don't do this. Even x86 will crash if you touch it with
> >>> something like an XMM optimized memcpy.
> >>>
> >>> Christian? If the vmap succeeds what rules must the caller use to
> >>> access the memory?
> >> See dma-buf-map.h and especially struct dma_buf_map.
> >>
> >> MMIO memory is perfectly supported here and actually the most common case.
> > Okay that looks sane, but this rxe RFC seems to ignore this
> > completely. It stuffs the vaddr directly into a umem which goes to all
> > manner of places in the driver.
> >
> > ??
>
> Well, yes that can go boom pretty quickly.
Sorry, I was wrong. The dma_buf_map treats both iomem and vaddr region, but
this RFC only supports vaddr. Advantage of the partial support is we can use the
vaddr dma-buf in RXE without changing a rxe data copy implementation.

An example of a dma-buf pointing to a vaddr is some gpu drivers use RAM for
VRAM and we can get dma-buf for the region that indicates vaddr regions.
Specifically, the gpu driver using gpu/drm/drm_gem_cma_helper.c is one such
example.

> Not sure what they want to use this for.
I'd like to use RDMA with RXE for that memory region.

Best,
Shunsuke
> Christian.
>
> >
> > Jason
>