Re: [PATCH v2 4/4] vfio/pci: Allow MMIO regions to be exported through dma-buf

[Dan Williams <dan.j.williams@intel.com>]

Christoph Hellwig wrote:
> On Wed, Sep 07, 2022 at 09:33:11AM -0300, Jason Gunthorpe wrote:
> > Yes, you said that, and I said that when the AMD driver first merged
> > it - but it went in anyhow and now people are using it in a bunch of
> > places.
> 
> drm folks made up their own weird rules, if they internally stick
> to it they have to listen to it given that they ignore review comments,
> but it violates the scatterlist API and has not business anywhere
> else in the kernel.  And yes, there probably is a reason or two why
> the drm code is unusually error prone.
> 
> > > Why would small BARs be problematic for the pages?  The pages are more
> > > a problem for gigantic BARs do the memory overhead.
> > 
> > How do I get a struct page * for a 4k BAR in vfio?
> 
> I guess we have different definitions of small then :)
> 
> But unless my understanding of the code is out out of data,
> memremap_pages just requires the (virtual) start address to be 2MB
> aligned, not the size.  Adding Dan for comments.

The minimum granularity for sparse_add_section() that memremap_pages
uses internally is 2MB, so start and end need to be 2MB aligned. Details
here:

ba72b4c8cf60 mm/sparsemem: support sub-section hotplug