Re: [PATCH v5 0/5] x86/cpufeatures: Automatically generate required and disabled feature masks

[Xin Li]

On 1/5/2025 11:07 PM, 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@xxxxxxxxx/.
> 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@xxxxxxx/.
> https://lore.kernel.org/all/20240126041126.1927228-23-michael.roth@xxxxxxx/.
>
> 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@xxxxxxxx/.
>
> 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.
>
> Recently when working on the immediate form of MSR access instructions,
> I needed to add a new CPU feature word for CPUID.7.1.ECX, and I had to
> replace the same check "(NCAPINTS != 22)" with (NCAPINTS != 23) in 3
> different files as
> https://github.com/xinli-intel/linux-fred-public/commit/aa80536927fcd293be8ae54e1d5e4d886cf83f21
> Obviously these replacements could be saved if the patch to add a new
> CPU feature word is reworked on top of this patch set.
>
> So this seems a good opportunity to demonstrate the convenience out of
> this patch set, and here comes v5 with just one more patch on top of v4
> that adds a new CPU feature word for CPUID.7.1.ECX and uses it for the
> immediate form of MSR access feature.
>
> Link to v4:
> https://lore.kernel.org/lkml/20240628174544.3118826-1-xin@xxxxxxxxx/

The patch proposed in the following review reminds me this infra
improvement patch set again, so a gentle ping :)

https://lore.kernel.org/lkml/CA+i-1C3LDJTf26WdBo7MvCdTs-dybqsUp3Ze7NowcOUHPWMXrg@xxxxxxxxxxxxxx/

Thanks!
    Xin