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

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Message-ID: <BN9PR11MB5276A08041ED09F4B215D50F8CAA2@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Mon, 7 Apr 2025 05:16:23 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Wathsala Wathawana Vithanage <wathsala.vithanage@....com>, Alex Williamson
	<alex.williamson@...hat.com>
CC: Jason Gunthorpe <jgg@...pe.ca>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, nd <nd@....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>, "Nagarahalli, Honnappa"
	<Honnappa.Nagarahalli@....com>, Jeremy Linton <Jeremy.Linton@....com>
Subject: RE: [RFC PATCH] vfio/pci: add PCIe TPH to device feature ioctl

> From: Wathsala Wathawana Vithanage <wathsala.vithanage@....com>
> Sent: Friday, March 14, 2025 9:49 AM
> > >
> > > 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.
> > >
> >
> > PCI capabilities are managed by the kernel VFIO driver. So w/o this
> > patch no userspace driver can enable TPH to try that trick?
> 
> Yes, it's possible. It's just a matter of setting the right bits in the PCI config
> space to enable TPH on the device.
> 

No. Before this patch the TPH capability is read-only to userspace,
enforced by vfio-pci. So there is no way that an user can toggle
the TPH cap by itself!