linux-kernel - RE: [PATCH] hv: add CONFIG

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Message-ID: <20250610163647.17887-1-romank@linux.microsoft.com>
Date: Tue, 10 Jun 2025 09:36:47 -0700
From: Roman Kisel <romank@...ux.microsoft.com>
To: mhklinux@...look.com
Cc: arnd@...db.de,
	arnd@...nel.org,
	decui@...rosoft.com,
	haiyangz@...rosoft.com,
	kys@...rosoft.com,
	linux-hyperv@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	nunodasneves@...ux.microsoft.com,
	romank@...ux.microsoft.com,
	ssengar@...ux.microsoft.com,
	wei.liu@...nel.org
Subject: RE: [PATCH] hv: add CONFIG_EFI dependency

>the VTL2 paravisor is supported only in Generation 2 VMs. But I'm not clear on
>what dependencies on EFI the VTL2 paravisor might have, if any. Roman -- are
>VTL2 paravisors built with CONFIG_EFI=n?

Thank you, Michael, for your help! We disable EFI:
* https://github.com/microsoft/OHCL-Linux-Kernel/blob/product/hcl-main/6.12/Microsoft/hcl-arm64.config
* https://github.com/microsoft/OHCL-Linux-Kernel/blob/product/hcl-main/6.12/Microsoft/hcl-x4.config

>
>> >>
>> >
>> > Hyper-V as of recent can boot off DeviceTree with the direct kernel  boot, no UEFI
>> > is required (examples would be OpenVMM and the OpenHCL paravisor on arm64).
>> 
>> I was aware of hyperv no longer needing ACPI, but devicetree and UEFI
>> are orthogonal concepts, and I had expected that even the devicetree
>> based version would still get booted using a tiny UEFI implementation
>> even if the kernel doesn't need that. Do you know what type of bootloader
>> is actually used in the examples you mentioned? Does the hypervisor
>> just start the kernel at the native entry point without a bootloader
>> in this case?
>
>Need Roman to clarify this.
>

The kernel is started at the native entry point in "Image", and the address of
the DeviceTree blob is passed in X0.

There is a "virtual baremetal" bootloader [1] that prepares DeviceTree for the
kernel, sets some CPU registers and jumps to the Image's entry point yet that's
totally headless except for the optional serial output. No ACPI or UEFI is involved
at all. For all intents and purposes, that's the kernel direct boot :)

[1]
https://github.com/microsoft/openvmm/tree/main/openhcl/openhcl_boot