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

[Wei Chen <Wei.Chen@arm.com>]

Hi Jan,

> -----Original Message-----
> From: Jan Beulich <jbeulich@suse.com>
> Sent: 2022年1月27日 17:17
> To: Wei Chen <Wei.Chen@arm.com>
> Cc: Bertrand Marquis <Bertrand.Marquis@arm.com>; xen-
> devel@lists.xenproject.org; sstabellini@kernel.org; julien@xen.org
> Subject: Re: [PATCH 20/37] xen: introduce CONFIG_EFI to stub API for non-
> EFI architecture
> 
> On 27.01.2022 10:09, Wei Chen wrote:
> >> From: Jan Beulich <jbeulich@suse.com>
> >> Sent: 2022年1月27日 17:00
> >>
> >> On 27.01.2022 09:51, Wei Chen wrote:
> >>>> From: Xen-devel <xen-devel-bounces@lists.xenproject.org> On Behalf Of
> >> Wei
> >>>> Chen
> >>>> Sent: 2022年1月27日 16:45
> >>>>
> >>>>> From: Jan Beulich <jbeulich@suse.com>
> >>>>> Sent: 2022年1月25日 18:35
> >>>>>
> >>>>> On 23.09.2021 14:02, Wei Chen wrote:
> >>>>>> --- a/xen/common/Kconfig
> >>>>>> +++ b/xen/common/Kconfig
> >>>>>> @@ -11,6 +11,16 @@ config COMPAT
> >>>>>>  config CORE_PARKING
> >>>>>>  	bool
> >>>>>>
> >>>>>> +config EFI
> >>>>>> +	bool
> >>>>>> +	---help---
> >>>>>> +      This option provides support for runtime services provided
> >>>>>> +      by UEFI firmware (such as non-volatile variables, realtime
> >>>>>> +      clock, and platform reset). A UEFI stub is also provided to
> >>>>>> +      allow the kernel to be booted as an EFI application. This
> >>>>>> +      is only useful for kernels that may run on systems that have
> >>>>>> +      UEFI firmware.
> >>>>>
> >>>>> The way enabling of (full) EFI support works on x86, I consider it
> >>>>> wrong / misleading to put the option in common code. At the very
> least
> >>>>> the help text would need to call out the extra dependencies. Plus
> the
> >>>>> help text of course then needs to be generic (i.e. applicable to
> both
> >>>>> Arm and x86). That's notwithstanding the fact that without a prompt
> >>>>> the help text won't ever be seen while configuring Xen.
> >>>>>
> >>>>> Also (nit): Indentation. And please don't use ---help--- anymore in
> >>>>> new code.
> >>>>>
> >>>>
> >>>> I have used CONFIG_ARM_EFI to replace this common EFI config in my
> >>>> latest version. This Kconfig option has been removed.
> >>>> And thanks, I will not use --help-- anymore.
> >>>>
> >>>>>> --- a/xen/include/xen/efi.h
> >>>>>> +++ b/xen/include/xen/efi.h
> >>>>>> @@ -25,6 +25,8 @@ extern struct efi efi;
> >>>>>>
> >>>>>>  #ifndef __ASSEMBLY__
> >>>>>>
> >>>>>> +#ifdef CONFIG_EFI
> >>>>>> +
> >>>>>>  union xenpf_efi_info;
> >>>>>>  union compat_pf_efi_info;
> >>>>>>
> >>>>>> @@ -45,6 +47,8 @@ int efi_runtime_call(struct
> xenpf_efi_runtime_call
> >>>> *);
> >>>>>>  int efi_compat_get_info(uint32_t idx, union compat_pf_efi_info *);
> >>>>>>  int efi_compat_runtime_call(struct compat_pf_efi_runtime_call *);
> >>>>>>
> >>>>>> +#endif /* CONFIG_EFI*/
> >>>>>
> >>>>> I can see that in the later patch, when introducing inline stubs,
> >>>>> you would need conditionals here, but I don't think you need them
> >>>>> right here (or you may want to introduce the stubs right away).
> >>>>>
> >>>>> Also (nit): Missing blank in the comment.
> >>>
> >>> I am sorry, I had missed this comment. In my latest changes,
> >>> I have introduced a stub file for non-EFI architectures.
> >>> The reason why we don't use a macro to stub the helpers
> >>> in efi.h is that, some architectures have implemented stub
> >>> helpers in their stub.c. If we define stub helpers in
> >>> efi.h, this will cause function redefinition error. We need
> >>> to fix this error for all architectures. And some helpers
> >>> is not easy to implement as a inline function in efi.h.
> >>> So we use stub file instead of stubing in efi.h
> >>
> >> But you realize we already have such a stub file on x86?
> >>
> >
> > Yes, we found the redefinition errors that are caused by x86 stub file
> > and new macros in stub.h. We had tries to add:
> > ifeq ($(XEN_BUILD_EFI),y)
> > CFLAGS-y += -DXEN_BUILD_EFI
> > XEN_CFLAGS += -DXEN_BUILD_EFI
> > endif
> > x86/Makefile to gate these new macros, but it seems that we may need
> > to change EFI build logic for x86. It will cause more risks for me.
> > So I want to introduce a similar stub.c in arch/arm.
> 
> While that's perhaps fine, ideally common bits would be common. Iirc
> already back at the time I was wondering why stub.c had to be x86-
> only.

Some dummy functions in stub.c can be shared by arm or other architectures.
But some functions like efi_multiboot2 are architecture dependent.

> 
> Jan