Re: [PATCH] PCI/P2PDMA: Root complex whitelist should not apply when an IOMMU is present

[Logan Gunthorpe <logang@deltatee.com>]

On 2019-05-24 6:40 a.m., Koenig, Christian wrote:
> Am 23.05.19 um 17:59 schrieb Christoph Hellwig:
>> [CAUTION: External Email]
>>
>> On Thu, May 23, 2019 at 09:53:53AM -0600, Logan Gunthorpe wrote:
>>>> The problem shows up if pci_bus_address() returns a different address
>>>> than pci_resource_start(), should be easy to check if that happens.
>>>> IIRC it is something mostly seen on embedded SOCs.
>>>>
>>> I think it's a bit more complicated then that: If you're calling
>>> dma_map_resource() to program the IOMMU then I'm pretty sure you'd want
>>> to use the pci_resource_start() address as the phys_addr_t. If you're
>>> bypassing the root complex (like the current p2pdma code enforces), then
>>> you'd simply use a pci_bus_address() directly as the dma_addr and would
>>> not program the IOMMU at all seeing it's not involved (which is what is
>>> currently done).
>> True.  What we need is:
>>
>>   if (both device are behind the same root port (using a switch)) {
>>          use the current direct map + offset code
>>   } else {
>>          call ->map_resource()
>>   }
> 
> Sounds sane to me as well.
> 
> But since I don't have a struct pages backing my PCI BAR I won't be able 
> to use pci_p2pdma_map_sg.
> 
> How should we work around that?

I'd add something like:

int pci_p2pdma_map_resource(struct pci_dev *provider, struct device
*mapper, phys_addr_t addr, size_t size,  enum dma_data_direction dir,
unsigned long attrs);

But keep in mind we have to update pci_p2pdma_map_sg() as well before we
can enable the whitelist as there are existing users. The map_sg()
command should be able to get the provider device through the pgmap. And
that will be easier to do once Dan's patch[1] lands because he creates a
private pgmap struct.

Logan

[1]
https://lore.kernel.org/lkml/155727338646.292046.9922678317501435597.stgit@dwillia2-desk3.amr.corp.intel.com/T/#u