On Tue, 4 Mar 2025 22:38:16 +0000 Wathsala Wathawana Vithanage <wathsala.vithanage@xxxxxxx> wrote: > > > Linux v6.13 introduced the PCIe TLP Processing Hints (TPH) feature for > > > direct cache injection. As described in the relevant patch set [1], > > > direct cache injection in supported hardware allows optimal platform > > > resource utilization for specific requests on the PCIe bus. This feature > > > is currently available only for kernel device drivers. However, > > > user space applications, especially those whose performance is sensitive > > > to the latency of inbound writes as seen by a CPU core, may benefit from > > > using this information (E.g., DPDK cache stashing RFC [2] or an HPC > > > application running in a VM). > > > > > > This patch enables configuring of TPH from the user space via > > > VFIO_DEVICE_FEATURE IOCLT. It provides an interface to user space > > > drivers and VMMs to enable/disable the TPH feature on PCIe devices and > > > set steering tags in MSI-X or steering-tag table entries using > > > VFIO_DEVICE_FEATURE_SET flag or read steering tags from the kernel using > > > VFIO_DEVICE_FEATURE_GET to operate in device-specific mode. > > > > What level of protection do we expect to have here? Is it OK for > > userspace to make up any old tag value or is there some security > > concern with that? > > > Shouldn't be allowed from within a container. > A hypervisor should have its own STs and map them to platform STs for > the cores the VM is pinned to and verify any old ST is not written to the > device MSI-X, ST table or device specific locations. And how exactly are we mediating device specific steering tags when we don't know where/how they're written to the device. An API that returns a valid ST to userspace doesn't provide any guarantees relative to what userspace later writes. MSI-X tables are also writable by userspace. I could have missed it, but I also didn't note any pinning requirement in this proposal. Thanks, Alex
