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

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Message-ID: <Z_TIMh7UsWQiSkqg@gmail.com>
Date: Tue, 8 Apr 2025 08:54:42 +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:

> 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?

Thanks,

	Ingo

View attachment "config" of type "text/plain" (138619 bytes)