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) > > This is particularly suited for the case (which used to exist, I don't > > know if it still does) where the buffer that wants write combining > > reside in the same BAR as registers that otherwise don't. > > IIUC that's still the case for some devices (I think Jason mentioned > some Mellanox cards). Right, VFIO will have to allow it page-by-page > I think this interface would help KVM when we'll need a cacheable > mapping. For WC, we are ok without any VFIO changes. Yes, it may be interesting to map cachable CXL memory as NORMAL_NC into userspace for similar reasons. Jason
