linux-kernel - RE: [PATCH hyperv-next v5 03/11] Drivers: hv: Enable VTL mode for arm64

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Message-ID:
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Date: Mon, 10 Mar 2025 21:01:42 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Arnd Bergmann <arnd@...db.de>, Roman Kisel <romank@...ux.microsoft.com>,
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>, Borislav Petkov <bp@...en8.de>,
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 Anvin" <hpa@...or.com>, Joey Gouly <joey.gouly@....com>, "krzk+dt@...nel.org"
	<krzk+dt@...nel.org>, Krzysztof Wilczyński <kw@...ux.com>,
	"K. Y. Srinivasan" <kys@...rosoft.com>, Len Brown <lenb@...nel.org>, Lorenzo
 Pieralisi <lpieralisi@...nel.org>, Manivannan Sadhasivam
	<manivannan.sadhasivam@...aro.org>, Mark Rutland <mark.rutland@....com>, Marc
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	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-hyperv@...r.kernel.org"
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CC: "apais@...rosoft.com" <apais@...rosoft.com>, "benhill@...rosoft.com"
	<benhill@...rosoft.com>, "bperkins@...rosoft.com" <bperkins@...rosoft.com>,
	"sunilmut@...rosoft.com" <sunilmut@...rosoft.com>
Subject: RE: [PATCH hyperv-next v5 03/11] Drivers: hv: Enable VTL mode for
 arm64

From: Arnd Bergmann <arnd@...db.de> Sent: Saturday, March 8, 2025 1:05 PM
> 
> On Fri, Mar 7, 2025, at 23:02, Roman Kisel wrote:
> > @@ -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