Messages in this thread Patch in this message | | | From | Borislav Petkov <> | Subject | [PATCH] x86/cpu: Start documenting what the X86_FEATURE_ flag testing macros do | Date | Mon, 7 Nov 2022 22:15:05 +0100 |
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From: Borislav Petkov <bp@suse.de>
... and how and when they should be used.
This keeps popping up everytime people start poking at the CPU features testing machinery - which has admittedly grown some warts and would need cleaning up - or when they are wondering what function/macro to use.
Start documenting it first. Proper cleanup will follow once all the functionality has been agreed upon.
No functional changes.
Signed-off-by: Borislav Petkov <bp@suse.de> Link: https://lore.kernel.org/r/20220718141123.136106-3-mlevitsk@redhat.com --- arch/x86/include/asm/cpufeature.h | 21 ++++++++++++++------- 1 file changed, 14 insertions(+), 7 deletions(-)
diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h index 1a85e1fb0922..47ff025e7387 100644 --- a/arch/x86/include/asm/cpufeature.h +++ b/arch/x86/include/asm/cpufeature.h @@ -131,12 +131,13 @@ extern const char * const x86_bug_flags[NBUGINTS*32]; (unsigned long __percpu *)&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 - * is not relevant. + * This is the preferred macro to use when testing X86_FEATURE_ bits + * support without the need to test on a particular CPU but rather + * system-wide. It takes into account build-time disabled feature + * support too. All those macros mirror the kernel's idea of enabled + * CPU features and not necessarily what real, hardware CPUID bits are + * set or clear. For that use tools/arch/x86/kcpuid/ and/or potentially + * extend if it's feature list is lacking. */ #define cpu_feature_enabled(bit) \ (__builtin_constant_p(bit) && DISABLED_MASK_BIT_SET(bit) ? 0 : static_cpu_has(bit)) @@ -145,9 +146,15 @@ extern const char * const x86_bug_flags[NBUGINTS*32]; #define set_cpu_cap(c, bit) set_bit(bit, (unsigned long *)((c)->x86_capability)) -extern void setup_clear_cpu_cap(unsigned int bit); extern void clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int bit); +/* + * The setup_* prefixed variants enable/disable feature bits on all + * CPUs in the system and are used to replicate those settings before + * apply_forced_caps() has synthesized enabled and disabled bits across + * every CPU. + */ +extern void setup_clear_cpu_cap(unsigned int bit); #define setup_force_cpu_cap(bit) do { \ set_cpu_cap(&boot_cpu_data, bit); \ set_bit(bit, (unsigned long *)cpu_caps_set); \ -- 2.35.1
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