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

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Message-ID: <f9642a9f-27d4-4f84-818c-390194b898bf@linuxfoundation.org>
Date: Tue, 8 Apr 2025 10:51:50 -0600
From: Shuah Khan <skhan@...uxfoundation.org>
To: Ingo Molnar <mingo@...nel.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, Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH] x86/Kconfig: Fix dependency for X86_DEBUG_FPU

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
View attachment "config-6.14.0+" of type "text/plain" (215757 bytes)