linux-kernel - Re: [PATCH RFC 20/25] PCI/LUO: Avoid write to liveupdate devices at boot

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Message-ID: <20250731150132.GV26511@ziepe.ca>
Date: Thu, 31 Jul 2025 12:01:32 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Chris Li <chrisl@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Danilo Krummrich <dakr@...nel.org>, Len Brown <lenb@...nel.org>,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-acpi@...r.kernel.org, David Matlack <dmatlack@...gle.com>,
	Pasha Tatashin <tatashin@...gle.com>,
	Jason Miu <jasonmiu@...gle.com>, Vipin Sharma <vipinsh@...gle.com>,
	Saeed Mahameed <saeedm@...dia.com>,
	Adithya Jayachandran <ajayachandra@...dia.com>,
	Parav Pandit <parav@...dia.com>, William Tu <witu@...dia.com>,
	Mike Rapoport <rppt@...nel.org>, Leon Romanovsky <leon@...nel.org>
Subject: Re: [PATCH RFC 20/25] PCI/LUO: Avoid write to liveupdate devices at
 boot

On Tue, Jul 29, 2025 at 06:51:27PM -0700, Chris Li wrote:

> They follow a pattern that the original kernel needs to write to the
> device and change the device state. The liveupdate device needs to
> maintain the previous state not changed, therefore needs to prevent
> such write initialization in liveupdate case.

No, I fundamentally reject this position and your testing methodology.

The new kernel *should* be writing to config space and it *should* be
doing things like clearing and gaining control over MSI. It is fully
wrong to be blocking it like you are doing just to satify some
incorrect qemu based test checking for no config access.

Only some config accesse are bad. Each and every "bad" one needs to be
clearly explained *why* it is bad and only then mitigated.

Most mitigation are far harder than just if'ing around the config
write. My ATS/PASID/etc example for instance.

Jason