Re: [RESEND v1 2/3] x86/cpufeatures: Generate a feature mask header based on build config

[Xin Li]

On 6/17/2024 6:46 AM, H. Peter Anvin wrote:
> On June 16, 2024 12:26:48 AM PDT, Xin Li <xin@xxxxxxxxx> wrote:
> > On 5/9/2024 1:53 PM, Xin Li (Intel) wrote:
> > > From: "H. Peter Anvin (Intel)" <hpa@xxxxxxxxx>
> > >
> > > Introduce an AWK script to auto-generate a header with required and
> > > disabled feature masks based on <asm/cpufeatures.h> and current build
> > > config. Thus for any CPU feature with a build config, e.g., X86_FRED,
> > > simply add
> >
> > ...
> >
> > > +
> > > +		printf "\n#define %s_MASKS ", s;
> > > +		pfx = "{";
> > > +		for (i = 0; i < ncapints; i++) {
> > > +			printf "%s \\\n\t%s_MASK_%d", pfx, s, i;
> > > +			pfx = ",";
> > > +		}
> > > +		printf " \\\n}\n\n";
> > > +
> > > +		printf "#define %s_FEATURE(x) \\\n", s;
> > > +		printf "\t((( ";
> > > +		for (i = 0; i < ncapints; i++) {
> > > +			if (masks[i]) {
> > > +				printf "\\\n\t\t((x) >> 5) == %2d ? %s_MASK%-3d : ", i, s, i;
> > > +			}
> > > +		}
> > > +		printf "0 \\\n";
> > > +		printf "\t) >> ((x) & 31)) & 1)\n\n";
> >
> > This code generates 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.
> >
> > But they are NOT currently used, so I prefer to remove them for now.
> >
> > Thanks!
> >     Xin
>
> The goal with these is that it can eliminate the handwritten code that tests a long list of masks. Again, automation.

This piece of code is added back in the last patch of v3:

https://lore.kernel.org/lkml/20240622171435.3725548-5-xin@xxxxxxxxx/

The generated macros {REQUIRED|DISABLED}_MASK_BIT_SET are way shorter
than that in the original form:

#define REQUIRED_FEATURE(x)                             \
        ((                                              \
                ((x) >> 5) ==  0 ? REQUIRED_MASK0 :     \
                ((x) >> 5) ==  1 ? REQUIRED_MASK1 :     \
                ((x) >> 5) ==  3 ? REQUIRED_MASK3 : 0   \
        ) & (1 << ((x) & 31)))

#define REQUIRED_MASK_BIT_SET(x)                        \
        (REQUIRED_FEATURE(x) || BUILD_BUG_ON_ZERO(NCAPINTS != 22))

Thanks!
    Xin