From: Arnd Bergmann <arnd@arndb.de> Sent: Monday, March 10, 2025 2:21 PM

On Mon, Mar 10, 2025, at 22:01, Michael Kelley wrote:

quoted

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

quoted

quoted

 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.

It sounds like the latter is the real problem: enabling a feature
should never prevent something else from working. Can you describe
what VTL context is and why it requires an exception to a rather
fundamental rule here? If you build a kernel that runs on every
single piece of arm64 hardware and every hypervisor, why can't
you add HYPERV_VTL_MODE to that as an option?

VTL = Virtual Trust Level, and VSM = Virtual Secure Mode, are Hyper-V's
terminology for offering multiple execution environments with
hierarchical trust in the context of a single VM. A normal guest
operating system runs at VTL 0, and there are no other VTLs in use.
But in some environments, additional software may run as a paravisor
layer between the normal guest OS and the hypervisor. This software
runs at some other VTL > 0, and has a higher privilege level within
the VM than software running at VTL 0 (which is the lowest privilege).
VTL 2 is used today in the Azure cloud with CoCo VMs to run a
paravisor, and there may be other uses in the future. See [1] if you
want more details on VSM and VTLs. Also [2] for the CoCo VM use
case.

Ideally, a Linux kernel image could detect at runtime what VTL it is
running at, and "do the right thing". Unfortunately, on x86 Linux this
has proved difficult (or perhaps impossible) because the amount of
boot-time setup required to ask the question about the current VTL
is significant. The idiosyncrasies and historical baggage of x86 requires
that Linux do some x86-specific initialization steps for VTL > 0
before the question can be asked. Hence the introduction of
CONFIG_HYPERV_VTL_MODE, and the behavior that when it is
selected, the kernel image won't run normally in VTL 0.

I'll go out on a limb and say that I suspect on arm64 a runtime
determination based on querying the VTL *could* be made (though
I'm not the person writing the code). But taking advantage of that
on arm64 produces an undesirable dichotomy with x86.

Roman may have further thoughts on the topic, but that's
what I know about how we got here.

Michael

[1] https://learn.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/vsm
[2] https://techcommunity.microsoft.com/blog/windowsosplatform/openhcl-the-new-open-source-paravisor/4273172