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From: "Christian König" <christian.koenig@amd.com>
To: 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>,
	"Xiong, Jianxin" <jianxin.xiong@intel.com>
Subject: Re: [RFC PATCH v2 0/3] RDMA: add dma-buf support
Date: Thu, 2 Jul 2020 16:50:32 +0200
Message-ID: <11e93282-25da-841d-9be6-38b0c9703d42@amd.com> (raw)
In-Reply-To: <20200702132953.GS25301@ziepe.ca>

Am 02.07.20 um 15:29 schrieb Jason Gunthorpe:
> On Thu, Jul 02, 2020 at 03:10:00PM +0200, Daniel Vetter wrote:
>> On Wed, Jul 01, 2020 at 02:15:24PM -0300, Jason Gunthorpe wrote:
>>> On Wed, Jul 01, 2020 at 05:42:21PM +0200, Daniel Vetter wrote:
>>>>>>> All you need is the ability to stop wait for ongoing accesses to end and
>>>>>>> make sure that new ones grab a new mapping.
>>>>>> Swap and flush isn't a general HW ability either..
>>>>>>
>>>>>> I'm unclear how this could be useful, it is guarenteed to corrupt
>>>>>> in-progress writes?
>>>>>>
>>>>>> Did you mean pause, swap and resume? That's ODP.
>>>>> Yes, something like this. And good to know, never heard of ODP.
>>>> Hm I thought ODP was full hw page faults at an individual page
>>>> level,
>>> Yes
>>>
>>>> and this stop&resume is for the entire nic. Under the hood both apply
>>>> back-pressure on the network if a transmission can't be received,
>>>> but
>>> NIC's don't do stop and resume, blocking the Rx pipe is very
>>> problematic and performance destroying.
>>>
>>> The strategy for something like ODP is more complex, and so far no NIC
>>> has deployed it at any granularity larger than per-page.
>>>
>>>> So since Jason really doesn't like dma_fence much I think for rdma
>>>> synchronous it is. And it shouldn't really matter, since waiting for a
>>>> small transaction to complete at rdma wire speed isn't really that
>>>> long an operation.
>>> Even if DMA fence were to somehow be involved, how would it look?
>> Well above you're saying it would be performance destroying, but let's
>> pretend that's not a problem :-) Also, I have no clue about rdma, so this
>> is really just the flow we have on the gpu side.
> I see, no, this is not workable, the command flow in RDMA is not at
> all like GPU - what you are a proposing is a global 'stop the whole
> chip' Tx and Rx flows for an undetermined time. Not feasible
>
> What we can do is use ODP techniques and pause only the MR attached to
> the DMA buf with the process you outline below. This is not so hard to
> implement.

Well it boils down to only two requirements:

1. You can stop accessing the memory or addresses exported by the DMA-buf.

2. Before the next access you need to acquire a new mapping.

How you do this is perfectly up to you. E.g. you can stop everything, 
just prevent access to this DMA-buf, or just pause the users of this 
DMA-buf....

>
>> 3. rdma driver worker gets busy to restart rx:
>> 	1. lock all dma-buf that are currently in use (dma_resv_lock).
>> 	thanks to ww_mutex deadlock avoidance this is possible
> Why all? Why not just lock the one that was invalidated to restore the
> mappings? That is some artifact of the GPU approach?

No, but you must make sure that mapping one doesn't invalidate others 
you need.

Otherwise you can end up in a nice live lock :)

> And why is this done with work queues and locking instead of a
> callback saying the buffer is valid again?

You can do this as well, but a work queue is usually easier to handle 
than a notification in an interrupt context of a foreign driver.

Regards,
Christian.

>
> Jason

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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       ` 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 [this message]
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
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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