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

[Christoph Hellwig <hch@infradead.org>]

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.

That being said, what is the point of mapping say a 4k BAR for p2p?
You're not going to save a measurable amount of CPU overhead if that
is the only place you transfer to.