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

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

Posted by Borislav Petkov 3 weeks, 3 days ago

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.

https://people.kernel.org/tglx/notes-about-netiquette

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

Posted by Dave Hansen 2 weeks, 5 days ago

On 10/31/24 03:34, Borislav Petkov wrote:
> cpu_feature_enabled() should be used in most cases when CPU feature
> support needs to be tested in code. Document that.

Yes, please.  BTW, I know the code generation isn't great in some cases.
 But this is the right _way_ to call things no "boot_" or &boot_cpu_data
for system-wide things.  Underlying code generation can continue to be
fixed up over time.

Acked-by: Dave Hansen <dave.hansen@linux.intel.com>

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

Posted by Sohil Mehta 3 weeks, 3 days ago

On 10/31/2024 3:34 AM, Borislav Petkov wrote:
> 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(-)
> 

Looks good (a minor nit below),
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>


> 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.
> + *

Does "default CPU feature testing macro" roll better than "default CPU
features testing macro"?

> + * 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.
>   */

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

Posted by Borislav Petkov 3 weeks, 3 days ago

On October 31, 2024 7:26:27 PM GMT+01:00, Sohil Mehta <sohil.mehta@intel.com> wrote:
>Does "default CPU feature testing macro" roll better than "default CPU
>features testing macro"?

Waaay too finicky to me. No one cares, I'd say. 🤗😂

-- 
Sent from a small device: formatting sucks and brevity is inevitable.