linux-kernel - Re: [PATCH] x86/Kconfig: Fix dependency for X86_DEBUG

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Message-ID: <Z_bnzs3IPyhVIYaT@gmail.com>
Date: Wed, 9 Apr 2025 23:34:06 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Shuah Khan <skhan@...uxfoundation.org>
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, x86@...nel.org,
	linux-kernel@...r.kernel.org, hpa@...or.com
Subject: Re: [PATCH] x86/Kconfig: Fix dependency for X86_DEBUG_FPU


* Shuah Khan <skhan@...uxfoundation.org> wrote:

> On 4/8/25 00:54, Ingo Molnar wrote:
> > 
> > * Shuah Khan <skhan@...uxfoundation.org> wrote:
> > 
> > > Compile fails when X86_DEBUG_FPU is enabled and X86_REQUIRED_FEATURE_FPU is
> > > disabled. Add explicit dependency on X86_REQUIRED_FEATURE_FPU to fix it.
> > > 
> > > ../arch/x86/kernel/fpu/regset.c:411:(.text+0x4cf2f): undefined reference to `fpregs_soft_get'
> > > ld: vmlinux.o: in function `fpregs_set':
> > > ../arch/x86/kernel/fpu/regset.c:445:(.text+0x4d0da): undefined reference to `fpregs_soft_set'
> > > ld: vmlinux.o: in function `copy_fpstate_to_sigframe':
> > > ../arch/x86/kernel/fpu/signal.c:197:(.text+0x4da98): undefined reference to `fpregs_soft_get'
> > > 
> > 
> > So I cannot reproduce this build failure on either v6.14 or v6.15-rc1:
> > 
> >    starship:~/tip> git describe
> >    v6.15-rc1
> > 
> >    starship:~/tip> grep -E 'X86_32=|M486SX=|X86_REQUIRED_FEATURE|X86_DEBUG_FPU' .config
> >    CONFIG_X86_32=y
> >    CONFIG_M486SX=y
> >    CONFIG_X86_REQUIRED_FEATURE_ALWAYS=y
> >    CONFIG_X86_DEBUG_FPU=y
> > 
> >    starship:~/tip> make -j128 ARCH=i386 bzImage modules
> > 
> >    ...
> >    Kernel: arch/x86/boot/bzImage is ready  (#5)
> > 
> > > Signed-off-by: Shuah Khan <skhan@...uxfoundation.org>
> > > ---
> > > 6.14 compile worked without X86_REQUIRED_FEATURE_FPU enabled. Might be a
> > > new dependency. Enabling X86_REQUIRED_FEATURE_FPU fixed it for me on my
> > > test system.
> > 
> > So vanilla v6.14 does not have X86_REQUIRED_FEATURE Kconfig flags, at
> > all:
> > 
> >    starship:~/tip> grep FPU .config
> >    CONFIG_ARCH_HAS_KERNEL_FPU_SUPPORT=y
> >    CONFIG_X86_DEBUG_FPU=y
> >    # CONFIG_TEST_FPU is not set
> > 
> > Because X86_REQUIRED_FEATURE_FPU is a new v6.15-rc1 feature, introduced
> > via:
> > 
> >    3d37d9396eb3 ("x86/cpufeatures: Add {REQUIRED,DISABLED} feature configs")
> > 
> > I have no doubt you are seeing this build failure - but I think there
> > might be some other .config detail required to reproduce it, not
> > mentioned in your changelog. Could you please send the config you used?
> > 
> 
> Config attached.
> 
> grep _FPU /boot/config-6.14.0+
> CONFIG_ARCH_HAS_KERNEL_FPU_SUPPORT=y
> CONFIG_X86_DEBUG_FPU=y
> # CONFIG_TEST_FPU is not set
> 
> I noticed you are building ARCH=i386 - can you reproduce this
> with x86_64? That is what I am building.
> 
> thanks,
> -- Shuah

> #
> # Automatically generated file; DO NOT EDIT.
> # Linux/x86 6.14.0 Kernel Configuration
> #
> CONFIG_CC_VERSION_TEXT="gcc (Debian 14.2.0-17) 14.2.0"

Is this a vanilla v6.14 kernel? Because on upstream v6.14 'make 
oldconfig' gives me:

  Generate BTF type information (DEBUG_INFO_BTF) [N/y/?] (NEW) 

Maybe different build environment?

... and after answering that with 'N' the build succeeds:

  LD      arch/x86/boot/setup.elf
  OBJCOPY arch/x86/boot/setup.bin
  BUILD   arch/x86/boot/bzImage
Kernel: arch/x86/boot/bzImage is ready  (#3)

More importantly, X86_REQUIRED_FEATURE_FPU *does not exist* in the 
vanilla v6.14 kernel, it's a new v6.15 feature. So this part of your 
changelog totally doesn't apply to a v6.14 kernel:

> > > Compile fails when X86_DEBUG_FPU is enabled and X86_REQUIRED_FEATURE_FPU is
> > > disabled. Add explicit dependency on X86_REQUIRED_FEATURE_FPU to fix it.

Thanks,

	Ingo