On Mon, 2023-07-17 at 12:35 -0600, Alex Williamson wrote: > On Sun, 16 Jul 2023 19:30:23 -0300 > Jason Gunthorpe <jgg@xxxxxxxxxx> wrote: > > > On Sun, Jul 16, 2023 at 08:09:02AM -0700, Catalin Marinas wrote: > > > > > In terms of security for arm64 at least, Device vs Normal NC (or nc vs > > > wc in Linux terminology) doesn't make much difference with the former > > > occasionally being worse. The kernel would probably trust the DPDK code > > > if it allows direct device access. > > > > RDMA and DRM already allow device drivers to map WC to userspace on > > demand, we expect the platform to support this. > > > > > > So the userspace component needs to be responsible for selecting the > > > > mapping, the same way using the PCI sysfs resource files today allows > > > > to do that by selecting the _wc variant. > > > > > > I guess the sysfs interface is just trying to work around the VFIO > > > limitations. > > > > I think just nobody has ever asked for VFIO WC support. The main > > non-VM user is DPDK and none of the NIC drivers have wanted this (DPDK > > applications areis more of throughput than latency focused typically) > > Yes, QEMU can't know whether the device or driver want a WC BAR > mapping, so we've left it for KVM manipulation relative to VM use > cases. Nobody has followed through with a complete proposal to enable > it otherwise for direct userspace driver access, but I don't think > there's opposition to providing such a thing. Thanks, Ok, this is really backburner work for me but I'll try to cook up a POC patch in the near (hopefully) future along the lines of the subregions I proposed and we can discuss from there. Cheers, Ben.
