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.
