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

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Message-ID: <20250305185806.GB354403@ziepe.ca>
Date: Wed, 5 Mar 2025 14:58:06 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Wathsala Wathawana Vithanage <wathsala.vithanage@....com>
Cc: Alex Williamson <alex.williamson@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	nd <nd@....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>,
	Dhruv Tripathi <Dhruv.Tripathi@....com>,
	Honnappa Nagarahalli <Honnappa.Nagarahalli@....com>,
	Jeremy Linton <Jeremy.Linton@....com>
Subject: Re: [RFC PATCH] vfio/pci: add PCIe TPH to device feature ioctl

On Wed, Mar 05, 2025 at 06:11:22AM +0000, Wathsala Wathawana Vithanage wrote:

> By not enabling TPH in device-specific mode, hypervisors can ensure that
> setting an ST in a device-specific location (like queue contexts) will have no
> effect. VMs should also not be allowed to enable TPH. 

So many workloads run inside VMs now for security reasons that is not
a reasonable approach.

> I believe this could
> be enforced by trapping (causing VM exits) on MSI-X/ST table writes. 

Yes, I think this was always part of the plan for virtualization when
using a MSI-X table.

> Having said that, regardless of this proposal or the availability of kernel
> TPH support, a VFIO driver could enable TPH and set an arbitrary ST on the
> MSI-X/ST table or a device-specific location on supported platforms. If the
> driver doesn't have a list of valid STs, it can enumerate 8- or 16-bit STs and
> measure access latencies to determine valid ones.

And you think it is absolutely true that no TPH value can cause a
platform malfunction or security failure?

Jason