RE: [PATCH 20/37] xen: introduce CONFIG_EFI to stub API for non-EFI architecture

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 27 Sep 2021, Wei Chen wrote:
> > -----Original Message-----
> > From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of Wei
> > Chen
> > Sent: 2021年9月26日 18:25
> > To: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; julien@xxxxxxx; Bertrand Marquis
> > <Bertrand.Marquis@xxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > Subject: RE: [PATCH 20/37] xen: introduce CONFIG_EFI to stub API for non-
> > EFI architecture
> > 
> > Hi Jan,
> > 
> > > -----Original Message-----
> > > From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of
> > Jan
> > > Beulich
> > > Sent: 2021年9月24日 18:49
> > > To: Wei Chen <Wei.Chen@xxxxxxx>
> > > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; julien@xxxxxxx; Bertrand Marquis
> > > <Bertrand.Marquis@xxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > > Subject: Re: [PATCH 20/37] xen: introduce CONFIG_EFI to stub API for
> > non-
> > > EFI architecture
> > >
> > > On 24.09.2021 12:31, Wei Chen wrote:
> > > >> From: Jan Beulich <jbeulich@xxxxxxxx>
> > > >> Sent: 2021年9月24日 15:59
> > > >>
> > > >> On 24.09.2021 06:34, Wei Chen wrote:
> > > >>>> From: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > > >>>> Sent: 2021年9月24日 9:15
> > > >>>>
> > > >>>> On Thu, 23 Sep 2021, Wei Chen wrote:
> > > >>>>> --- a/xen/common/Kconfig
> > > >>>>> +++ b/xen/common/Kconfig
> > > >>>>> @@ -11,6 +11,16 @@ config COMPAT
> > > >>>>>  config CORE_PARKING
> > > >>>>>     bool
> > > >>>>>
> > > >>>>> +config EFI
> > > >>>>> +   bool
> > > >>>>
> > > >>>> Without the title the option is not user-selectable (or de-
> > > selectable).
> > > >>>> So the help message below can never be seen.
> > > >>>>
> > > >>>> Either add a title, e.g.:
> > > >>>>
> > > >>>> bool "EFI support"
> > > >>>>
> > > >>>> Or fully make the option a silent option by removing the help text.
> > > >>>
> > > >>> OK, in current Xen code, EFI is unconditionally compiled. Before
> > > >>> we change related code, I prefer to remove the help text.
> > > >>
> > > >> But that's not true: At least on x86 EFI gets compiled depending on
> > > >> tool chain capabilities. Ultimately we may indeed want a user
> > > >> selectable option here, but until then I'm afraid having this option
> > > >> at all may be misleading on x86.
> > > >>
> > > >
> > > > I check the build scripts, yes, you're right. For x86, EFI is not a
> > > > selectable option in Kconfig. I agree with you, we can't use Kconfig
> > > > system to decide to enable EFI build for x86 or not.
> > > >
> > > > So how about we just use this EFI option for Arm only? Because on Arm,
> > > > we do not have such toolchain dependency.
> > >
> > > To be honest - don't know. That's because I don't know what you want
> > > to use the option for subsequently.
> > >
> > 
> > In last version, I had introduced an arch-helper to stub EFI_BOOT
> > in Arm's common code for Arm32. Because Arm32 doesn't support EFI.
> > So Julien suggested me to introduce a CONFIG_EFI option for non-EFI
> > supported architectures to stub in EFI layer.
> > 
> > [1] https://lists.xenproject.org/archives/html/xen-devel/2021-
> > 08/msg00808.html
> > 
> 
> As Jan' reminded, x86 doesn't depend on Kconfig to build EFI code.
> So, if we CONFIG_EFI to stub EFI API's for x86, we will encounter
> that toolchains enable EFI, but Kconfig disable EFI. Or Kconfig
> enable EFI but toolchain doesn't provide EFI build supports. And
> then x86 could not work well.
> 
> If we use CONFIG_EFI for Arm only, that means CONFIG_EFI for x86
> is off, this will also cause problem.
> 
> So, can we still use previous arch_helpers to stub for Arm32?
> until x86 can use this selectable option?

EFI doesn't have to be necessarily a user-visible option in Kconfig at
this point. I think Julien was just asking to make the #ifdef based on
a EFI-related config rather than just based CONFIG_ARM64.

On x86 EFI is detected based on compiler support, setting XEN_BUILD_EFI
in xen/arch/x86/Makefile. Let's say that we keep using the same name
"XEN_BUILD_EFI" on ARM as well.

On ARM32, XEN_BUILD_EFI should be always unset.

On ARM64 XEN_BUILD_EFI should be always set.

That's it, right? I'd argue that CONFIG_EFI or HAS_EFI are better names
than XEN_BUILD_EFI, but that's OK anyway. So for instance you can make
XEN_BUILD_EFI an invisible symbol in xen/arch/arm/Kconfig and select it
only on ARM64.