[PATCH REGRESSION] um: Fix cpu_feature_enabled() build breakage

[Fabian Franz <fabian@isomorphic-ai.com>]

Since commit 3ed403bbc967 ("treewide: Remove CLOCK_TICK_RATE") deleted
the (otherwise empty) arch/um/include/asm/timex.h, UML builds resolve
<asm/timex.h> to arch/x86/include/asm/timex.h via the HEADER_ARCH
include path.  That pulls <asm/tsc.h>, whose get_cycles() uses
cpu_feature_enabled(), into virtually every translation unit, starting
with asm-offsets.c.  This newly expands parts of UML's copy of
<asm/cpufeature.h> that no longer compile, and "make ARCH=um" now dies
in prepare0:

  arch/um/include/asm/cpufeature.h:52:39: error: implicit declaration
    of function 'DISABLED_MASK_BIT_SET'
  arch/um/include/asm/cpufeature.h:115:17: error: implicit declaration
    of function '_static_cpu_has'; did you mean '__static_cpu_has'?

Two independent breakages meet here:

 1. cpu_feature_enabled() and this_cpu_has() still test
    DISABLED_MASK_BIT_SET()/REQUIRED_MASK_BIT_SET().  Those used to
    come from <asm/disabled-features.h>/<asm/required-features.h> via
    <asm/cpufeatures.h>, but since commit 8f97566c8a81
    ("x86/cpufeatures: Remove {disabled,required}-features.h") they
    live in the generated <asm/cpufeaturemasks.h>, which only the
    arch/x86 archprepare rule generates.  ARCH=um never generates nor
    includes it, so the references have been dangling since then.

 2. The _static_cpu_has() macro expands to itself.  Commit
    3eaa50e1e255 ("x86/cpu: Hide and rename static_cpu_has()") renamed
    the inline function _static_cpu_has() -> __static_cpu_has() and
    the macro static_cpu_has() -> _static_cpu_has(), but in the UML
    header the macro's out-of-line branch kept calling
    _static_cpu_has(), which now names the macro itself and is left
    unexpanded by the preprocessor.

Fix the macro to call __static_cpu_has(), as the x86 header does, and
drop the mask based short-circuits from cpu_feature_enabled() and
this_cpu_has().  UML has no compile-time feature masking to express:
capabilities are copied from the host's CPUID at boot, see commit
d8fb32f4790f ("um: Add support for host CPU flags and alignment"),
and generating cpufeaturemasks.h from a UML .config would wrongly
mark features disabled simply because the gating CONFIG_X86_* symbols
do not exist for ARCH=um.

Fixes: 3eaa50e1e255 ("x86/cpu: Hide and rename static_cpu_has()")
Fixes: 8f97566c8a81 ("x86/cpufeatures: Remove {disabled,required}-features.h")
Signed-off-by: Fabian Franz <fabian@isomorphic-ai.com>
Assisted-By: Claude Fable 5 <noreply@anthropic.com>
---
 arch/um/include/asm/cpufeature.h | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/arch/um/include/asm/cpufeature.h b/arch/um/include/asm/cpufeature.h
index f7770083c0a4..20b2c7ead62e 100644
--- a/arch/um/include/asm/cpufeature.h
+++ b/arch/um/include/asm/cpufeature.h
@@ -37,8 +37,7 @@ extern const char * const x86_bug_flags[NBUGINTS*32];
 	 test_cpu_cap(c, bit)
 
 #define this_cpu_has(bit)						\
-	(__builtin_constant_p(bit) && REQUIRED_MASK_BIT_SET(bit) ? 1 :	\
-	 x86_this_cpu_test_bit(bit, cpu_info.x86_capability))
+	 x86_this_cpu_test_bit(bit, cpu_info.x86_capability)
 
 /*
  * This macro is for detection of features which need kernel
@@ -48,8 +47,7 @@ extern const char * const x86_bug_flags[NBUGINTS*32];
  * supporting a possible guest feature where host support for it
  * is not relevant.
  */
-#define cpu_feature_enabled(bit)	\
-	(__builtin_constant_p(bit) && DISABLED_MASK_BIT_SET(bit) ? 0 : _static_cpu_has(bit))
+#define cpu_feature_enabled(bit)	_static_cpu_has(bit)
 
 #define boot_cpu_has(bit)	cpu_has(&boot_cpu_data, bit)
 
@@ -112,7 +110,7 @@ static __always_inline bool __static_cpu_has(u16 bit)
 (								\
 	__builtin_constant_p(boot_cpu_has(bit)) ?		\
 		boot_cpu_has(bit) :				\
-		_static_cpu_has(bit)				\
+		__static_cpu_has(bit)				\
 )
 
 #define cpu_has_bug(c, bit)		cpu_has(c, (bit))
-- 
2.43.0

Every ARCH=um build has failed since the 7.3 merge window.

#regzbot introduced: 3ed403bbc967

Steps to reproduce on master:

make ARCH=um O=/tmp/um-build defconfig
make ARCH=um O=/tmp/um-build -j$(nproc)

fails with:

/home/ubuntu/projects/linux/arch/x86/include/asm/tsc.h: In function ‘get_cycles’:
/home/ubuntu/projects/linux/arch/um/include/asm/cpufeature.h:52:39: error: implicit declaration of function ‘DISABLED_MASK_BIT_SET’ [-Werror=implicit-function-declaration]
   52 |         (__builtin_constant_p(bit) && DISABLED_MASK_BIT_SET(bit) ? 0 : _static_cpu_has(bit))
      |                                       ^~~~~~~~~~~~~~~~~~~~~
/home/ubuntu/projects/linux/arch/x86/include/asm/tsc.h:79:14: note: in expansion of macro ‘cpu_feature_enabled’
   79 |         if (!cpu_feature_enabled(X86_FEATURE_TSC))
      |              ^~~~~~~~~~~~~~~~~~~
/home/ubuntu/projects/linux/arch/um/include/asm/cpufeature.h:115:17: error: implicit declaration of function ‘_static_cpu_has’; did you mean ‘__static_cpu_has’? [-Werror=implicit-function-declaration]
  115 |                 _static_cpu_has(bit)                            \
      |                 ^~~~~~~~~~~~~~~
/home/ubuntu/projects/linux/arch/um/include/asm/cpufeature.h:52:72: note: in expansion of macro ‘_static_cpu_has’
   52 | builtin_constant_p(bit) && DISABLED_MASK_BIT_SET(bit) ? 0 : _static_cpu_has(bit))
      |                                                             ^~~~~~~~~~~~~~~

If this is known please disregard. I am also not 100% sure if this is the right fix or if it's better to bring back the macros to um.

In any case it would be good for um to build again on the master branch out of the box.

Thanks,
Fabian