Re: [Xen-devel] [XEN PATCH 6/8] xen: Move CONFIG_INDIRECT_THUNK to Kconfig

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Fri, Dec 13, 2019 at 12:13:53PM +0100, Jan Beulich wrote:
> On 12.12.2019 19:27, Anthony PERARD wrote:
> > --- a/xen/arch/x86/Kconfig
> > +++ b/xen/arch/x86/Kconfig
> > @@ -32,6 +32,9 @@ config ARCH_DEFCONFIG
> >     string
> >     default "arch/x86/configs/x86_64_defconfig"
> >  
> > +config INDIRECT_THUNK
> > +   def_bool $(cc-option,-mindirect-branch-register)
> 
> I'm not happy to see constructs like this appear. They leave a
> "# CONFIG_... is not defined" in .config for no reason when
> the expression evaluates to n.

For some reason, this doesn't happen. If $(CC) doesn't understand the
option, the CONFIG_ doesn't appear at all in .config.

I guess "# CONFIG_... is not defined" comments are only useful for
options that can be selected by users, so Kconfig would know not to ask
the users again. So, for "options" that aren't for users, the comment
doesn't need to exist.

> This may not matter much when
> considering just one such line, but it will when we gain
> dozens or hundreds. For options without prompts I think the
> default should only ever be set to y (or m, which we don't
> use). The above would then be written as

I think Kconfig developers have already done the work for that :-). So
the construct def_bool y if $X isn't needed. Maybe I should read to doc
about the difference between "def_bool n" and "def_bool y if n", but
there is probably no difference.

> config INDIRECT_THUNK
>       def_bool y if $(cc-option,-mindirect-branch-register)

Thanks,

-- 
Anthony PERARD

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