Re: [PATCH 0/8] dma-mapping: migrate to physical address-based API

Jason Gunthorpe
2025-08-05T15:36:12Z

On Mon, Aug 04, 2025 at 04:37:56AM +0100, Matthew Wilcox wrote:
> On Sun, Aug 03, 2025 at 12:59:06PM -0300, Jason Gunthorpe wrote:
> > Matthew, do you think it makes sense to introduce types to make this
> > clearer? We have two kinds of values that a phys_addr_t can store -
> > something compatible with kmap_XX_phys(), and something that isn't.
>
> I was with you up until this point. And then you said "What if we have
> a raccoon that isn't a raccoon" and my brain derailed.

I though it was clear..

kmap_local_pfn(phys >> PAGE_SHIFT)
phys_to_virt(phys)

Does not work for all values of phys. It definately illegal for
non-cachable MMIO. Agree?

There is a subset of phys that is cachable and has struct page that is
usable with kmap_local_pfn()/etc

phys is always this:

> - CPU untranslated. This is the "physical" address. Physical address
> 0 is what the CPU sees when it drives zeroes on the memory bus.

But that is a pure HW perspective. It doesn't say which of our SW APIs
are allowed to use this address.

We have callchains in DMA API land that want to do a kmap at the
bottom. It would be nice to mark the whole call chain that the
phys_addr being passed around is actually required to be kmappable.

Because if you pass a non-kmappable MMIO backed phys it will explode
in some way on some platforms.

> > We clearly have these two different ideas floating around in code,
> > page tables, etc.

> No. No, we don't. I've never heard of this asininity before.

Welcome to the fun world of cachable and non-cachable memory.

Consider, today we can create struct pages of type
MEMORY_DEVICE_PCI_P2PDMA for non-cachable MMIO. I think today you
"can" use kmap to establish a cachable mapping in the vmap.

But it is *illegal* to establish a cachable CPU mapping of MMIO. Archs
are free to MCE if you do this - speculative cache line load of MMIO
can just error in HW inside the interconnect.

So, the phys_addr is always a "CPU untranslated physical address" but
the cachable/non-cachable cases, or DRAM vs MMIO, are sometimes
semantically very different things for the SW!

Jason