From: Daniel Vetter <daniel@ffwll.ch>
To: "Christian König" <christian.koenig@amd.com>
Cc: Leon Romanovsky <leon@kernel.org>,
"linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>,
"dri-devel@lists.freedesktop.org"
<dri-devel@lists.freedesktop.org>, Jason Gunthorpe <jgg@ziepe.ca>,
Doug Ledford <dledford@redhat.com>,
"Vetter, Daniel" <daniel.vetter@intel.com>,
"Xiong, Jianxin" <jianxin.xiong@intel.com>
Subject: Re: [RFC PATCH v2 0/3] RDMA: add dma-buf support
Date: Wed, 8 Jul 2020 11:49:34 +0200 [thread overview]
Message-ID: <20200708094934.GI3278063@phenom.ffwll.local> (raw)
In-Reply-To: <d28286c7-b1c1-a4a8-1d38-264ed1761cdd@amd.com>
On Wed, Jul 08, 2020 at 11:38:31AM +0200, Christian König wrote:
> Am 07.07.20 um 23:58 schrieb Xiong, Jianxin:
> > > -----Original Message-----
> > > From: Christian König <christian.koenig@amd.com>
> > > Am 03.07.20 um 15:14 schrieb Jason Gunthorpe:
> > > > On Fri, Jul 03, 2020 at 02:52:03PM +0200, Daniel Vetter wrote:
> > > >
> > > > > So maybe I'm just totally confused about the rdma model. I thought:
> > > > > - you bind a pile of memory for various transactions, that might
> > > > > happen whenever. Kernel driver doesn't have much if any insight into
> > > > > when memory isn't needed anymore. I think in the rdma world that's
> > > > > called registering memory, but not sure.
> > > > Sure, but once registered the memory is able to be used at any moment
> > > > with no visibilty from the kernel.
> > > >
> > > > Unlike GPU the transactions that trigger memory access do not go
> > > > through the kernel - so there is no ability to interrupt a command
> > > > flow and fiddle with mappings.
> > > This is the same for GPUs with user space queues as well.
> > >
> > > But we can still say for a process if that this process is using a DMA-buf which is moved out and so can't run any more unless the DMA-buf is
> > > accessible again.
> > >
> > > In other words you somehow need to make sure that the hardware is not accessing a piece of memory any more when you want to move it.
> > >
> > While a process can be easily suspended, there is no way to tell the RDMA NIC not to process posted work requests that use specific memory regions (or with any other conditions).
> >
> > So far it appears to me that DMA-buf dynamic mapping for RDMA is only viable with ODP support. For NICs without ODP, a way to allow pinning the device memory is still needed.
>
> And that's exactly the reason why I introduced explicit pin()/unpin()
> functions into the DMA-buf API:
> https://elixir.bootlin.com/linux/latest/source/drivers/dma-buf/dma-buf.c#L811
>
> It's just that at least our devices drivers currently prevent P2P with
> pinned DMA-buf's for two main reasons:
>
> a) To prevent deny of service attacks because P2P BARs are a rather rare
> resource.
>
> b) To prevent failures in configuration where P2P is not always possible
> between all devices which want to access a buffer.
So the above is more or less the question in the cover letter (which
didn't make it to dri-devel). Can we somehow throw that limitation out, or
is that simply not a good idea?
Simply moving buffers to system memory when they're pinned does simplify a
lot of headaches. For a specific custom built system we can avoid that
maybe, but I think upstream is kinda a different thing.
Cheers, Daniel
> Regards,
> Christian.
>
> >
> > Jianxin
> >
> > > Christian.
> > >
> > > > Jason
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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next prev parent reply other threads:[~2020-07-08 9:49 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <1593451903-30959-1-git-send-email-jianxin.xiong@intel.com>
[not found] ` <20200629185152.GD25301@ziepe.ca>
[not found] ` <MW3PR11MB4555A99038FA0CFC3ED80D3DE56F0@MW3PR11MB4555.namprd11.prod.outlook.com>
[not found] ` <20200630173435.GK25301@ziepe.ca>
2020-06-30 18:46 ` [RFC PATCH v2 0/3] RDMA: add dma-buf support Xiong, Jianxin
2020-06-30 19:17 ` Jason Gunthorpe
2020-06-30 20:08 ` Xiong, Jianxin
2020-07-02 12:27 ` Jason Gunthorpe
2020-07-01 9:03 ` Christian König
2020-07-01 12:07 ` Daniel Vetter
2020-07-01 12:14 ` Daniel Vetter
2020-07-01 12:39 ` Jason Gunthorpe
2020-07-01 12:55 ` Christian König
2020-07-01 15:42 ` Daniel Vetter
2020-07-01 17:15 ` Jason Gunthorpe
2020-07-02 13:10 ` Daniel Vetter
2020-07-02 13:29 ` Jason Gunthorpe
2020-07-02 14:50 ` Christian König
2020-07-02 18:15 ` Daniel Vetter
2020-07-03 12:03 ` Jason Gunthorpe
2020-07-03 12:52 ` Daniel Vetter
2020-07-03 13:14 ` Jason Gunthorpe
2020-07-03 13:21 ` Christian König
2020-07-07 21:58 ` Xiong, Jianxin
2020-07-08 9:38 ` Christian König
2020-07-08 9:49 ` Daniel Vetter [this message]
2020-07-08 14:20 ` Christian König
2020-07-08 14:33 ` Alex Deucher
2020-06-30 18:56 ` Xiong, Jianxin
[not found] ` <1593451903-30959-2-git-send-email-jianxin.xiong@intel.com>
2020-06-30 19:04 ` [RFC PATCH v2 1/3] RDMA/umem: Support importing dma-buf as user memory region Xiong, Jianxin
[not found] ` <1593451903-30959-3-git-send-email-jianxin.xiong@intel.com>
2020-06-30 19:04 ` [RFC PATCH v2 2/3] RDMA/core: Expand the driver method 'reg_user_mr' to support dma-buf Xiong, Jianxin
[not found] ` <1593451903-30959-4-git-send-email-jianxin.xiong@intel.com>
2020-06-30 19:05 ` [RFC PATCH v2 3/3] RDMA/uverbs: Add uverbs command for dma-buf based MR registration Xiong, Jianxin
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