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

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Message-ID:
 <SN6PR02MB4157E15EFE263BBA3D8DFC51D4EC2@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Wed, 15 May 2024 13:37:06 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Roman Kisel <romank@...ux.microsoft.com>, "arnd@...db.de" <arnd@...db.de>,
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>, "bp@...en8.de" <bp@...en8.de>,
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	<sunilmut@...rosoft.com>, "vdso@...bites.dev" <vdso@...bites.dev>
Subject: RE: [PATCH v2 2/6] drivers/hv: Enable VTL mode for arm64

From: Roman Kisel <romank@...ux.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@...ux.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