[PATCH] x86/cpufeature: Document cpu_feature_enabled() as the default to use

Borislav Petkov
2024-10-31T10:34:34Z

From: "Borislav Petkov (AMD)" <bp@alien8.de>

cpu_feature_enabled() should be used in most cases when CPU feature
support needs to be tested in code. Document that.

Reported-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
---
arch/x86/include/asm/cpufeature.h | 18 ++++++------------
1 file changed, 6 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index 0b9611da6c53..de1ad09fe8d7 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -132,11 +132,12 @@ extern const char * const x86_bug_flags[NBUGINTS*32];
x86_this_cpu_test_bit(bit, cpu_info.x86_capability))

/*
- * This macro is for detection of features which need kernel
- * infrastructure to be used. It may *not* directly test the CPU
- * itself. Use the cpu_has() family if you want true runtime
- * testing of CPU features, like in hypervisor code where you are
- * supporting a possible guest feature where host support for it
+ * This is the default CPU features testing macro to use in code.
+ *
+ * It is for detection of features which need kernel infrastructure to be
+ * used. It may *not* directly test the CPU itself. Use the cpu_has() family
+ * if you want true runtime testing of CPU features, like in hypervisor code
+ * where you are supporting a possible guest feature where host support for it
* is not relevant.
*/
#define cpu_feature_enabled(bit) \
@@ -161,13 +162,6 @@ extern void clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int bit);
#define setup_force_cpu_bug(bit) setup_force_cpu_cap(bit)

/*
- * Static testing of CPU features. Used the same as boot_cpu_has(). It
- * statically patches the target code for additional performance. Use
- * static_cpu_has() only in fast paths, where every cycle counts. Which
- * means that the boot_cpu_has() variant is already fast enough for the
- * majority of cases and you should stick to using it as it is generally
- * only two instructions: a RIP-relative MOV and a TEST.
- *
* Do not use an "m" constraint for [cap_byte] here: gcc doesn't know
* that this is only used on a fallback path and will sometimes cause
* it to manifest the address of boot_cpu_data in a register, fouling
--
2.43.0

--
Regards/Gruss,
Boris.