Re: [RFC PATCH v2 0/3] RDMA: add dma-buf support

From: "Christian König" <christian.koenig@amd.com>
To: "Xiong, Jianxin" <jianxin.xiong@intel.com>,
	Jason Gunthorpe <jgg@ziepe.ca>, Daniel Vetter <daniel@ffwll.ch>
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>,
	Doug Ledford <dledford@redhat.com>,
	"Vetter, Daniel" <daniel.vetter@intel.com>
Subject: Re: [RFC PATCH v2 0/3] RDMA: add dma-buf support
Date: Wed, 8 Jul 2020 11:38:31 +0200	[thread overview]
Message-ID: <d28286c7-b1c1-a4a8-1d38-264ed1761cdd@amd.com> (raw)
In-Reply-To: <MW3PR11MB45553DB31AD67C8B870A345FE5660@MW3PR11MB4555.namprd11.prod.outlook.com>

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.

Regards,
Christian.

>
> Jianxin
>
>> Christian.
>>
>>> Jason