Re: VFIO (PCI) and write combine mapping of BARs

[Benjamin Herrenschmidt]

On Fri, 2023-07-14 at 09:37 -0300, Jason Gunthorpe wrote:
> 
> There are two topics here
> 
> 1) Make ARM KVM allow the VM to select WC for its MMIO. This has
>    evolved in a way that is not related to VFIO
> 
> 2) Allow VFIO to create mmaps with WC for non-VM use cases like DPDK.
> 
> We have a draft patch for #1, and I think a general understanding with
> ARM folks that this is the right direction.
> 
> 2 is more like what this email talks about - providing mmaps with
> specific flags.
> 
> Benjamin, which are you interested in?

Sorry for the delay, got caught up.... The customer request we have
(and what I was indeed talking about) is 2. That said, when running in
a VM, 2 won't do much without 1.

> > > The problem isn't so much the low level implementation, we just have to
> > > play with the pgprot, the question is more around what API to present
> > > to control this.
> 
> Assuming this is for #2, I think VFIO has fallen into a bit of a trap
> by allowing userspace to form the mmap offset. I've seen this happen
> in other subsystems too. It seems like a good idea then you realize
> you need more stuff in the mmap space and become sad.
> 
> Typically the way out is to covert the mmap offset into a cookie where
> userspace issues some ioctl and then the ioctl returns an opaque mmap
> offset to use.
> 
> eg in the vfio context you'd do some 'prepare region for mmap' ioctl
> where you could specify flags. The kernel would encode the flags in
> the cookie and then mmap would do the right thing. Adding more stuff
> is done by enhancing the prepare ioctl.
> 
> Legacy mmap offsets are kept working.

This indeed what I have in mind. IE. VFIO has legacy regions and add-on
regions though the latter is currently only exploited by some drivers
that create their own add-on regions. My proposal is to add an ioctl to
create them from userspace as "children" of an existing driver-provided
region, allowing to set different attributes for mmap.

> > > This is still quite specific to PCI, but so is the entire regions
> > > mechanism, so I don't see an easy path to something more generic at
> > > this stage.
> 
> Regions are general, but the encoding of the mmap cookie has various
> PCI semantics when used with the PCI interface..
> 
> We'd want the same ability with platform devices too, for instance.

In the current VFIO the implementation is *entirely* in vfio_pci_core
for PCI and entirely in vfio_platform_common.c for platform, so while
the same ioctls could be imagined to create sub-regions, it would have
to be completely implemented twice unless we do a lot of heavy lifting
to move some of that region stuff into common code.

But yes, appart from that, no objection :-)

Cheers,
Ben.