RE: [PATCH v2 2/6] drivers/hv: Enable VTL mode for arm64

Michael Kelley
2024-05-15T13:37:13Z

From: Roman Kisel <romank@linux.microsoft.com> Sent: Tuesday, May 14, 2024 3:44 PM
>
> Kconfig dependencies for arm64 guests on Hyper-V require that be ACPI enabled,
> and limit VTL mode to x86/x64. To enable VTL mode on arm64 as well, update the
> dependencies. Since VTL mode requires DeviceTree instead of ACPI, don't require
> arm64 guests on Hyper-V to have ACPI.
>
> Signed-off-by: Roman Kisel <romank@linux.microsoft.com>
> ---
> drivers/hv/Kconfig | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig
> index 862c47b191af..a5cd1365e248 100644
> --- a/drivers/hv/Kconfig
> +++ b/drivers/hv/Kconfig
> @@ -5,7 +5,7 @@ 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)
> select PARAVIRT
> select X86_HV_CALLBACK_VECTOR if X86
> select OF_EARLY_FLATTREE if OF
> @@ -15,7 +15,7 @@ config HYPERV
>
> config HYPERV_VTL_MODE
> bool "Enable Linux to boot in VTL context"
> - depends on X86_64 && HYPERV
> + depends on HYPERV
> depends on SMP
> default n
> help

These changes make it possible to build a normal VTL 0 Hyper-V
guest (i.e., CONFIG_HYPERV_VTL_MODE=n) if CONFIG_ACPI is
not set, which won't work. While we can say "don't do that", it
would be better if the Kconfig dependencies expressed that
requirement.

A possible fix is to remove the "depends on HYPERV" from
HYPERV_VTL_MODE. Then for HYPERV, make
the "depends on ACPI" be conditional on !HYPERV_VTL_MODE
(for both ARM64 and X86).

I think we originally had "depends on HYPERV" in
HYPERV_VTL_MODE because there was a VTL-related function
in a non-Hyper-V code path, and we wanted to prevent that code
from running in non-Hyper-V environments. But in practice, that
turned out not to work well because occasionally people would
do an "all config" build where both CONFIG_HYPERV and
CONFIG_HYPERV_VTL_MODE were set, and it would panic during
boot in their non-Hyper-V environment. Such people were not
happy. :-( So Saurabh made a relatively simple change (see commit
14058f72cf13e) that got the VTL code out of that non-Hyper-V code
path. With that change, it shouldn't matter if someone sets
CONFIG_HYPERV_VTL_MODE=y in a build where
CONFIG_HYPERV=n.

At least that's my theory. :-) Someone would need to check
it carefully.

Michael