linux-kernel - Re: [PATCH 11/14] PCI/P2PDMA: dma_map P2PDMA map requests that traverse the host bridge

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Message-ID: <cf61d237-8a8a-e3ac-a9df-466f20b63020@deltatee.com>
Date:   Thu, 25 Jul 2019 13:17:02 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Christoph Hellwig <hch@....de>,
        Christian Koenig <Christian.Koenig@....com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Dan Williams <dan.j.williams@...el.com>,
        Eric Pilmore <epilmore@...aio.com>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [PATCH 11/14] PCI/P2PDMA: dma_map P2PDMA map requests that
 traverse the host bridge



On 2019-07-25 12:58 p.m., Jason Gunthorpe wrote:
> On Mon, Jul 22, 2019 at 05:08:56PM -0600, Logan Gunthorpe wrote:
>> Any requests that traverse the host bridge will need to be mapped into
>> the IOMMU, so call dma_map_sg() inside pci_p2pdma_map_sg() when
>> appropriate.
>>
>> Similarly, call dma_unmap_sg() inside pci_p2pdma_unmap_sg().
>>
>> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
>>  drivers/pci/p2pdma.c | 31 ++++++++++++++++++++++++++++++-
>>  1 file changed, 30 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
>> index 5f43f92f9336..76f51678342c 100644
>> +++ b/drivers/pci/p2pdma.c
>> @@ -830,8 +830,22 @@ int pci_p2pdma_map_sg_attrs(struct device *dev, struct scatterlist *sg,
>>  		int nents, enum dma_data_direction dir, unsigned long attrs)
>>  {
>>  	struct dev_pagemap *pgmap = sg_page(sg)->pgmap;
>> +	struct pci_dev *client;
>> +	int dist;
>> +
>> +	client = find_parent_pci_dev(dev);
>> +	if (WARN_ON_ONCE(!client))
>> +		return 0;
>>  
>> -	return __pci_p2pdma_map_sg(pgmap, dev, sg, nents);
>> +	dist = upstream_bridge_distance(pgmap->pci_p2pdma_provider,
>> +					client, NULL);
>> +	if (WARN_ON_ONCE(dist & P2PDMA_NOT_SUPPORTED))
>> +		return 0;
>> +
>> +	if (dist & P2PDMA_THRU_HOST_BRIDGE)
>> +		return dma_map_sg_attrs(dev, sg, nents, dir, attrs);
> 
> IIRC at this point the SG will have struct page references to the BAR
> memory - so (all?) the IOMMU drivers are able to handle P2P setup in
> this case?

Yes. The IOMMU drivers refer to the physical address for BAR which they
can get from the struct page. And this works fine today.

> Didn't you also have a patch to remove the struct page's for the BAR
> memory?  

Well that was an RFC that's not going anywhere in it's current form. I'd
still like to remove struct pages but that's going to take a while and
when that happens, this will have to be reworked. Probably to use
dma_map_resource() instead but will depend on the form the removal takes.

Logan