Re: [PATCH v4 0/4] x86/cpufeatures: Automatically generate required and disabled feature masks

[Xin Li <xin@zytor.com>]

On 6/28/2024 10:45 AM, Xin Li (Intel) wrote:
> The x86 build process first generates required and disabled feature
> masks based on current build config, and then uses these generated
> masks to compile the source code. When a CPU feature is not enabled
> in a build config, e.g., when CONFIG_X86_FRED=n, its feature disable
> flag, i.e., DISABLE_FRED, needs to be properly defined and added to
> a specific disabled CPU features mask in <asm/disabled-features.h>,
> as the following patch does:
> https://lore.kernel.org/all/20231205105030.8698-8-xin3.li@intel.com/.
> As a result, the FRED feature bit is surely cleared in the generated
> kernel binary when CONFIG_X86_FRED=n.
> 
> Recently there is another case to repeat the same exercise for the
> AMD SEV-SNP CPU feature:
> https://lore.kernel.org/all/20240126041126.1927228-2-michael.roth@amd.com/.
> https://lore.kernel.org/all/20240126041126.1927228-23-michael.roth@amd.com/.
> 
> It was one thing when there were four of CPU feature masks, but with
> over 20 it is going to cause mistakes, e.g.,
> https://lore.kernel.org/lkml/aaed79d5-d683-d1bc-7ba1-b33c8d6db618@suse.com/.
> 
> We want to eliminate the stupidly repeated exercise to manually assign
> features to CPU feature words through introducing an AWK script to
> automatically generate a header with required and disabled CPU feature
> masks based on current build config, and this patch set does that.
> 
> Link to v3:
> https://lore.kernel.org/lkml/20240622171435.3725548-1-xin@zytor.com/
> 
> Changes since v3:
> * Add a few high-level comments to the AWK script (Nikolay Borisov).
> * Use '1U' instead of '1' in feature mask shifting (Andrew Cooper).
> * Checking NCAPINTS isn't necessary anymore.  It was needed when these
>    macros had to be manually updated, but now if cpufeatures.h changes
>    this header will be regenerated (Brian Gerst).
> * Enforce CPU feature mask values to be unsigned.
> 
> Changes since v2:
> * Remove AWK code that generates extra debugging comments (Brian Gerst).
> * Move SSE_MASK to verify_cpu.S, the only place it is used (Brian Gerst).
> * Add a patch to generate macros {REQUIRED|DISABLED}_MASK_BIT_SET in the
>    new AWK script (Brian Gerst).
> 
> Changes since v1:
> * Keep the X86_{REQUIRED,DISABLED}_FEATURE_ prefixes solely in
>    arch/x86/Kconfig.cpufeatures (Borislav Petkov).
> * Explain how config option names X86_{REQUIRED,DISABLED}_FEATURE_<name>
>    are formed (Borislav Petkov).
> * Remove code generating unused macros {REQUIRED,DISABLED}_FEATURE(x)
>    to tell if a CPU feature, e.g., X86_FEATURE_FRED, is a required or
>    disabled feature for this particular compile-time configuration.
> 

A gentle ping...