Re: [PATCH] x86/Kconfig: Fix dependency for X86_DEBUG_FPU

[Shuah Khan]

On 4/9/25 15:34, Ingo Molnar wrote:
> * Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx> wrote:
>
> > On 4/8/25 00:54, Ingo Molnar wrote:
> > > * Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx>
> > > > ---
> > > > 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.

Right. That is what I meant by new dependency

> > > 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)

Yes it is vanilla

> 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:

I started with vanilla 6.14 kernel running oldconfig on it.
In this case if X86_DEBUG_FPU is enabled in the oldconfig,
should the config generated for 6.15 add X86_REQUIRED_FEATURE_FPU.

It appears there is a dependency between X86_DEBUG_FPU and
the newly added X86_REQUIRED_FEATURE_FPU

thanks,
-- Shuah