Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v5 06/13] PCI/P2PDMA: Add P2P DMA driver writer's
 documentation
To:     Logan Gunthorpe <logang@deltatee.com>,
        linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
        linux-nvme@lists.infradead.org, linux-rdma@vger.kernel.org,
        linux-nvdimm@lists.01.org, linux-block@vger.kernel.org
Cc:     Stephen Bates <sbates@raithlin.com>,
        Christoph Hellwig <hch@lst.de>,
        Keith Busch <keith.busch@intel.com>,
        Sagi Grimberg <sagi@grimberg.me>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Max Gurtovoy <maxg@mellanox.com>,
        Dan Williams <dan.j.williams@intel.com>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Alex Williamson <alex.williamson@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>
References: <20180830185352.3369-1-logang@deltatee.com>
 <20180830185352.3369-7-logang@deltatee.com>
 <98bff500-4e4c-3a34-6762-16ef4b076d90@amd.com>
 <6820d451-9591-1979-00bf-60c65fc4f136@deltatee.com>
From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>
Message-ID: <30411655-9b90-749e-857f-318dac7cccbf@amd.com>
Date:   Fri, 31 Aug 2018 19:38:50 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.9.1
MIME-Version: 1.0
In-Reply-To: <6820d451-9591-1979-00bf-60c65fc4f136@deltatee.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Received-SPF: None (protection.outlook.com: amd.com does not designate
 permitted sender hosts)
SpamDiagnosticOutput: 1:99
SpamDiagnosticMetadata: NSPM
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 31 Aug 2018 17:39:04.2977 (UTC)
X-MS-Exchange-CrossTenant-Network-Message-Id: b1c04268-80a0-4973-89c8-08d60f68a7d9
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d
X-MS-Exchange-Transport-CrossTenantHeadersStamped: CY4PR12MB1718
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

Am 31.08.2018 um 17:51 schrieb Logan Gunthorpe:
>
> On 31/08/18 02:08 AM, Christian König wrote:
>>> +One of the biggest issues is that PCI doesn't require forwarding
>>> +transactions between hierarchy domains, and in PCIe, each Root Port
>>> +defines a separate hierarchy domain. To make things worse, there is no
>>> +simple way to determine if a given Root Complex supports this or not.
>>> +(See PCIe r4.0, sec 1.3.1). Therefore, as of this writing, the kernel
>>> +only supports doing P2P when the endpoints involved are all behind the
>>> +same PCI bridge, as such devices are all in the same PCI hierarchy
>>> +domain, and the spec guarantees that all transacations within the
>>> +hierarchy will be routable, but it does not require routing
>>> +between hierarchies.
>> Can we add a kernel command line switch and a whitelist to enable P2P
>> between separate hierarchies?
> In future work, yes. But not for this patchset. This is definitely the
> way I see things going, but we've chosen to start with what we've presented.

Sounds like a plan to me.

If you can separate out adding the detection I can take a look adding 
this with my DMA-buf P2P efforts.

Christian.

>
> Logan