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

[Roman Kisel]

On 5/15/2024 6:37 AM, Michael Kelley wrote:
> From: Roman Kisel <romank@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx>
> > ---
> >   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.
I'll explore that, appreciate sharing the context!

> Michael

--
Thank you,
Roman