linux-kernel - Re: [PATCH] x86/of: Restore possibility to use both ACPI and FDT from bootloader

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Message-ID: <Z3wmGIOG-ge63Hce@google.com>
Date: Mon, 6 Jan 2025 19:51:04 +0100
From: Dmytro Maluka <dmaluka@...omium.org>
To: Christoph Hellwig <hch@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Rob Herring <robh@...nel.org>,
	Saravana Kannan <saravanak@...gle.com>,
	Saurabh Sengar <ssengar@...ux.microsoft.com>,
	Usama Arif <usamaarif642@...il.com>,
	Stephen Boyd <sboyd@...nel.org>,
	Frank Rowand <frowand.list@...il.com>,
	"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <linux-kernel@...r.kernel.org>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE" <devicetree@...r.kernel.org>,
	Pierre-Clément Tosi <ptosi@...gle.com>,
	Shikha Panwar <shikhapanwar@...gle.com>,
	Will Deacon <will@...nel.org>, Keir Fraser <keirf@...gle.com>,
	Michal Mazurek <mazurekm@...gle.com>,
	Bartłomiej Grzesik <bgrzesik@...gle.com>,
	Tomasz Nowicki <tnowicki@...gle.com>,
	Grzegorz Jaszczyk <jaszczyk@...gle.com>
Subject: Re: [PATCH] x86/of: Restore possibility to use both ACPI and FDT
 from bootloader

On Mon, Jan 06, 2025 at 03:16:15AM -0800, Christoph Hellwig wrote:
> On Mon, Dec 23, 2024 at 06:18:03PM +0000, Dmytro Maluka wrote:
> > There are cases when the bootloader provides information to the kernel
> > in both ACPI and DTB, not interchangeably. One such use case is virtual
> > machines in Android. When running on x86, the Android Virtualization
> > Framework (AVF) boots VMs with ACPI like it is usually done on x86 (i.e.
> > the virtual LAPIC, IOAPIC, HPET, PCI MMCONFIG etc are described in ACPI)
> > but also passes various AVF-specific boot parameters in DTB. This allows
> > reusing the same implementations of various AVF components on both
> > arm64 and x86.
> 
> What are these "AVF-specific boot parameters"?

Let me quote my reply to Rob:

The strictly AVF-specific properties are described in [1]. When it comes
to Linux guests, actually AFAIK currently all those properties are
consumed by the guest userspace, not the guest kernel, so the role of
the kernel is in fact just to pass them over to the userspace via sysfs.

Besides that, one important DT binding used by AVF's Linux guest
kernels, not userspace, is google,open-dice documented in
Documentation/devicetree/bindings/reserved-memory/google,open-dice.yaml.
[2] describes how it is used by the protected VM firmware (pvmfw) to
securely boot protected VMs.

Cc'ing some AVF folks to keep me honest.

(AVF used to support protected VMs on arm64 only, but now we are trying
to make them work on x86. So, taking google,open-dice as an example, we
could add an ACPI binding to the open-dice driver, however bloating
pvmfw with AML support is a no go, so we want to keep passing it via DT
on x86 as well.)

[1] https://android.googlesource.com/platform/packages/modules/Virtualization/+/refs/heads/main/docs/device_trees.md#dt-for-guests
[2] https://android.googlesource.com/platform/packages/modules/Virtualization/+/refs/heads/main/guest/pvmfw/#handover-abi