Re: [RFC PATCH] vfio/pci: add PCIe TPH to device feature ioctl

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 4 Mar 2025 22:38:16 +0000
Wathsala Wathawana Vithanage <wathsala.vithanage@arm.com> 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