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

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Message-ID: <20250304141447.GY5011@ziepe.ca>
Date: Tue, 4 Mar 2025 10:14:47 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Wathsala Vithanage <wathsala.vithanage@....com>
Cc: linux-kernel@...r.kernel.org, nd@....com,
	Alex Williamson <alex.williamson@...hat.com>,
	Kevin Tian <kevin.tian@...el.com>,
	Philipp Stanner <pstanner@...hat.com>,
	Yunxiang Li <Yunxiang.Li@....com>,
	"Dr. David Alan Gilbert" <linux@...blig.org>,
	Ankit Agrawal <ankita@...dia.com>,
	"open list:VFIO DRIVER" <kvm@...r.kernel.org>
Subject: Re: [RFC PATCH] vfio/pci: add PCIe TPH to device feature ioctl

On Fri, Feb 21, 2025 at 10:46:33PM +0000, Wathsala Vithanage 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?

Jason