Re: [PATCH] x86/config: Make the x86 defconfigs a bit more usable

Masahiro Yamada
2022-09-05T02:17:54Z

On Sun, Sep 4, 2022 at 6:48 PM Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> > > Unfortunately, even without the ARCH=i386 'make savedefconfig' doesn't
> > > seem to be doing the right thing & is dropping the '# CONFIG_64BIT is
> > > not set' line:
> >
> >
> > Oh, really?
> >
> > Without ARCH=i386, it works correctly for me.
> >
> >
> >
> > masahiro@zoe:~/ref/linux$ make i386_defconfig savedefconfig
> > #
> > # No change to .config
> > #
> > masahiro@zoe:~/ref/linux$ grep CONFIG_64BIT defconfig
> > # CONFIG_64BIT is not set
>
> Yeah, so why do these two seemingly identical commands produce two
> different .config's:
>
> $ make ARCH=i386 defconfig
> *** Default configuration is based on 'i386_defconfig'
>
> $ make i386_defconfig
>
> ?

As Nathan explained in this thread,
the prompt of CONFIG_64BIT is hidden
when ARCH=i386.

config 64BIT
bool "64-bit kernel" if "$(ARCH)" = "x86"
default "$(ARCH)" != "i386"
help
Say yes to build a 64-bit kernel - formerly known as x86_64
Say no to build a 32-bit kernel - formerly known as i386

[1] "make ARCH=i386 defconfig"

ARCH is i386, so the Kconfig snippet becomes as follows:

config 64BIT
bool
default n

[2] "make i386_defconfig"

ARCH is x86, so the Kconfig snippet becomes as follows:

config 64BIT
bool "64-bit kernel"
default y

"# ... is not set" is output when
the prompt is visible and the user did not enable it.

> Basically the canonical way to generate a defconfig is to provide an ARCH,
> then use the common 'defconfig' target:

I do not know what you mean by "canonical", but
there are only 3 architectures (x86, sparc, sh) that
allow two ways for configuration.

This is somewhat related to a historical reason
(commit 8c4d9b145ba3 mentioned this a little).

After arch/i386/ and arch/x86_64/ were merged into arch/x86,
ARCH=i386 and ARCH=x86_64 became a subset of ARCH=x86.

>
> make ARCH=i386 defconfig
> make ARCH=x86 defconfig
> make ARCH=arm defconfig
> make ARCH=arm64 defconfig
> make ARCH=powerpc defconfig
> make ARCH=s390 defconfig
> make ARCH=sparc defconfig
> make ARCH=sparc64 defconfig
> ...
>
> This is what my build test scripts are using to quick-test cross-arch
> builds. It's a straightforward way to generate defconfigs without knowing
> the details of the myriads of random defconfig targets some architectures
> have, such as:
>
> kepler:~/tip> ls arch/arm/configs/
> am200epdkit_defconfig gemini_defconfig multi_v5_defconfig s5pv210_defconfig
> aspeed_g4_defconfig h3600_defconfig multi_v7_defconfig sama5_defconfig
> aspeed_g5_defconfig h5000_defconfig mv78xx0_defconfig sama7_defconfig
> assabet_defconfig hackkit_defconfig mvebu_v5_defconfig shannon_defconfig
> at91_dt_defconfig hisi_defconfig mvebu_v7_defconfig shmobile_defconfig
> axm55xx_defconfig imxrt_defconfig mxs_defconfig simpad_defconfig
> badge4_defconfig imx_v4_v5_defconfig neponset_defconfig socfpga_defconfig
> bcm2835_defconfig imx_v6_v7_defconfig netwinder_defconfig sp7021_defconfig
> cerfcube_defconfig integrator_defconfig nhk8815_defconfig spear13xx_defconfig
> clps711x_defconfig iop32x_defconfig omap1_defconfig spear3xx_defconfig
> cm_x300_defconfig ixp4xx_defconfig omap2plus_defconfig spear6xx_defconfig
> cns3420vb_defconfig jornada720_defconfig orion5x_defconfig spitz_defconfig
> colibri_pxa270_defconfig keystone_defconfig oxnas_v6_defconfig stm32_defconfig
> colibri_pxa300_defconfig lart_defconfig palmz72_defconfig sunxi_defconfig
> collie_defconfig lpc18xx_defconfig pcm027_defconfig tct_hammer_defconfig
> corgi_defconfig lpc32xx_defconfig pleb_defconfig tegra_defconfig
> davinci_all_defconfig lpd270_defconfig pxa168_defconfig trizeps4_defconfig
> dove_defconfig lubbock_defconfig pxa255-idp_defconfig u8500_defconfig
> dram_0x00000000.config magician_defconfig pxa3xx_defconfig versatile_defconfig
> dram_0xc0000000.config mainstone_defconfig pxa910_defconfig vexpress_defconfig
> dram_0xd0000000.config milbeaut_m10v_defconfig pxa_defconfig vf610m4_defconfig
> ep93xx_defconfig mini2440_defconfig qcom_defconfig viper_defconfig
> eseries_pxa_defconfig mmp2_defconfig realview_defconfig vt8500_v6_v7_defconfig
> exynos_defconfig moxart_defconfig rpc_defconfig xcep_defconfig
> ezx_defconfig mps2_defconfig s3c2410_defconfig zeus_defconfig
> footbridge_defconfig multi_v4t_defconfig s3c6400_defconfig
>
> But this doesn't seem to be working reliably on i386:
>
> kepler:~/tip> make i386_defconfig
> #
> # configuration written to .config
> #
> kepler:~/tip> /bin/cp .config .config.i386_defconfig.1
>
> kepler:~/tip> make ARCH=i386 defconfig
> *** Default configuration is based on 'i386_defconfig'
> #
> # configuration written to .config
> #
> kepler:~/tip> /bin/cp .config .config.i386_defconfig.2
>
> kepler:~/tip> diff -up .config.i386_defconfig.1 .config.i386_defconfig.2
> --- .config.i386_defconfig.1 2022-09-04 11:34:48.253202205 +0200
> +++ .config.i386_defconfig.2 2022-09-04 11:35:04.268758331 +0200
> @@ -1,6 +1,6 @@
> #
> # Automatically generated file; DO NOT EDIT.
> -# Linux/x86 6.0.0-rc3 Kernel Configuration
> +# Linux/i386 6.0.0-rc3 Kernel Configuration
> #
> CONFIG_CC_VERSION_TEXT="gcc (Ubuntu 12-20220319-1ubuntu1) 12.0.1 20220319 (experimental) [master r12-7719-g8ca61ad148f]"
> CONFIG_CC_IS_GCC=y
> @@ -261,7 +261,6 @@ CONFIG_PROFILING=y
> CONFIG_TRACEPOINTS=y
> # end of General setup
>
> -# CONFIG_64BIT is not set
> CONFIG_X86_32=y
> CONFIG_X86=y
> CONFIG_INSTRUCTION_DECODER=y
>
> Note how in the ARCH=i386 case the build system claims to use i386_defconfig:
>
> *** Default configuration is based on 'i386_defconfig'
>
> But that doesn't seem to be identical with when we specify i386_defconfig
> directly as a target...
>
> What am I missing?
>
> Thanks,
>
> Ingo

--
Best Regards
Masahiro Yamada