Re: [PATCH V10 6/8] PCI/P2PDMA: Add a 10-Bit Tag check in P2PDMA

From: Logan Gunthorpe <logang@deltatee.com>
To: Bjorn Helgaas <helgaas@kernel.org>,
	Dongdong Liu <liudongdong3@huawei.com>
Cc: hch@infradead.org, kw@linux.com, leon@kernel.org,
	linux-pci@vger.kernel.org, rajur@chelsio.com,
	hverkuil-cisco@xs4all.nl, linux-media@vger.kernel.org,
	netdev@vger.kernel.org
Subject: Re: [PATCH V10 6/8] PCI/P2PDMA: Add a 10-Bit Tag check in P2PDMA
Date: Wed, 27 Oct 2021 17:41:07 -0600	[thread overview]
Message-ID: <136155cc-d28c-ef36-c69b-557f7af456be@deltatee.com> (raw)
In-Reply-To: <20211027231134.GA258571@bhelgaas>

On 2021-10-27 5:11 p.m., Bjorn Helgaas wrote:
>> @@ -532,6 +577,9 @@ calc_map_type_and_dist(struct pci_dev *provider, struct pci_dev *client,
>>  		map_type = PCI_P2PDMA_MAP_NOT_SUPPORTED;
>>  	}
>>  done:
>> +	if (pci_10bit_tags_unsupported(client, provider, verbose))
>> +		map_type = PCI_P2PDMA_MAP_NOT_SUPPORTED;
> 
> I need to be convinced that this check is in the right spot to catch
> all potential P2PDMA situations.  The pci_p2pmem_find() and
> pci_p2pdma_distance() interfaces eventually call
> calc_map_type_and_dist().  But those interfaces don't actually produce
> DMA bus addresses, and I'm not convinced that all P2PDMA users use
> them.
> 
> nvme *does* use them, but infiniband (rdma_rw_map_sg()) does not, and
> it calls pci_p2pdma_map_sg().

The rules of the current code is that calc_map_type_and_dist() must be
called before pci_p2pdma_map_sg(). The calc function caches the mapping
type in an xarray. If it was not called ahead of time,
pci_p2pdma_map_type() will return PCI_P2PDMA_MAP_NOT_SUPPORTED, and the
WARN_ON_ONCE will be hit in
pci_p2pdma_map_sg_attrs().

Both NVMe and RDMA (only used in the nvme fabrics code) do the correct
thing here and we can be sure calc_map_type_and_dist() is called before
any pages are mapped.

The patch set I'm currently working on will ensure that
calc_map_type_and_dist() is called before anyone maps a PCI P2PDMA page
with dma_map_sg*().

> amdgpu_dma_buf_attach() calls pci_p2pdma_distance_many() but I don't
> know where it sets up P2PDMA transactions.

The amdgpu driver hacked this in before proper support was done, but at
least it's using pci_p2pdma_distance_many() presumably before trying any
transfer. Though it's likely broken as it doesn't take into account the
mapping type and thus I think it always assumes traffic goes through the
host bridge (seeing it doesn't use pci_p2pdma_map_sg()).

> cxgb4 and qed mention "peer2peer", but I don't know whether they are
> related; they don't seem to use any pci_p2p.* interfaces.

I'm really not sure what these drivers are doing at all. However, I
think this is unrelated based on this old patch description[1]:

  Open MPI, Intel MPI and other applications don't support the iWARP
  requirement that the client side send the first RDMA message. This
  class of application connection setup is called peer-2-peer. Typically
  once the connection is setup, _both_ sides want to send data.

  This patch enables supporting peer-2-peer over the chelsio rnic by
  enforcing this iWARP requirement in the driver itself as part of RDMA
  connection setup.

Logan

[1] http://lkml.iu.edu/hypermail/linux/kernel/0804.3/1416.html