From: Arnd Bergmann <arnd@arndb.de> Sent: Saturday, March 8, 2025 1:05 PM

On Fri, Mar 7, 2025, at 23:02, Roman Kisel wrote:

quoted

@@ -5,18 +5,20 @@ menu "Microsoft Hyper-V guest support"
 config HYPERV
 	tristate "Microsoft Hyper-V client drivers"
 	depends on (X86 && X86_LOCAL_APIC && HYPERVISOR_GUEST) \
-		|| (ACPI && ARM64 && !CPU_BIG_ENDIAN)
+		|| (ARM64 && !CPU_BIG_ENDIAN)
+	depends on (ACPI || HYPERV_VTL_MODE)
 	select PARAVIRT
 	select X86_HV_CALLBACK_VECTOR if X86
-	select OF_EARLY_FLATTREE if OF
 	help
 	  Select this option to run Linux as a Hyper-V client operating
 	  system.

 config HYPERV_VTL_MODE
 	bool "Enable Linux to boot in VTL context"
-	depends on X86_64 && HYPERV
+	depends on (X86_64 || ARM64)
 	depends on SMP
+	select OF_EARLY_FLATTREE
+	select OF
 	default n
 	help

Having the dependency below the top-level Kconfig entry feels a little
counterintuitive. You could flip that back as it was before by doing

      select HYPERV_VTL_MODE if !ACPI
      depends on ACPI || SMP

in the HYPERV option, leaving the dependency on HYPERV in
HYPERV_VTL_MODE.

I would argue that we don't ever want to implicitly select
HYPERV_VTL_MODE because of some other config setting or
lack thereof.  VTL mode is enough of a special case that it should
only be explicitly selected. If someone omits ACPI, then HYPERV
should not be selectable unless HYPERV_VTL_MODE is explicitly
selected.

The last line of the comment for HYPERV_VTL_MODE says
"A kernel built with this option must run at VTL2, and will not run
as a normal guest."  In other words, don't choose this unless you
100% know that VTL2 is what you want.

Michael

Is OF_EARLY_FLATTREE actually needed on x86?

      Arnd