Re: [PATCH v7 10/13] nvme-pci: Add support for P2P memory in requests

[Keith Busch]

On Tue, Sep 25, 2018 at 11:41:44AM -0600, Logan Gunthorpe wrote:
> Hey,
> 
> On 2018-09-25 11:11 a.m., Keith Busch wrote:
> > Sorry if this was already discussed. Is there a reason the following
> > pattern is not pushed to the generic dma_map_sg_attrs?
> > 
> > 	if (is_pci_p2pdma_page(sg_page(sg)))
> > 		pci_p2pdma_map_sg(dev, sg, nents, dma_dir);
> > 
> > Beyond that, series looks good.
> 
> Yes, this has been discussed. It comes down to a few reasons:
> 
> 1) Intrusiveness on other systems: ie. not needing to pay the cost for
> every single dma_map_sg call
> 
> 2) Consistency: we can add the check to dma_map_sg, but adding similar
> functionality to dma_map_page, etc is difficult seeing it's hard for the
> unmap operation to detect if a dma_addr_t was P2P memory to begin with.
> 
> 3) Safety for developers trying to use P2P memory: Right now developers
> must be careful with P2P pages and ensure they aren't mapped using other
> means (ie dma_map_page). Having them check the drivers that are handling
> the pages to ensure the appropriate map function is always used is and
> that P2P pages aren't being mixed with regular pages is better than
> developers relying on magic in dma_map_sg() and getting things wrong.
> 
> That being said, I think in the future everyone would like to move in
> that direction but it means we will have to solve some difficult
> problems with the existing infrastructure.

Gotchya, thanks for jogging my memory.