linux-kernel - Re: [PATCH v3 2/4] x86/cpufeatures: Generate a feature mask header based on build config

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Date: Thu, 27 Jun 2024 09:45:34 -0700
From: Xin Li <xin@...or.com>
To: Nikolay Borisov <nik.borisov@...e.com>, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        will@...nel.org, peterz@...radead.org, akpm@...ux-foundation.org,
        acme@...nel.org, namhyung@...nel.org, brgerst@...il.com
Subject: Re: [PATCH v3 2/4] x86/cpufeatures: Generate a feature mask header
 based on build config

On 6/26/2024 4:36 AM, Nikolay Borisov wrote:
> 
> 
> On 22.06.24 г. 20:14 ч., Xin Li (Intel) wrote:
>> From: "H. Peter Anvin (Intel)" <hpa@...or.com>
>>
>> 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
>>
>> config X86_DISABLED_FEATURE_FRED
>>     def_bool y
>>     depends on !X86_FRED
>>
>> to arch/x86/Kconfig.cpufeatures, instead of adding a conditional CPU
>> feature disable flag, e.g., DISABLE_FRED.
>>
>> Lastly the generated required and disabled feature masks will be added
>> to their corresponding feature masks for this particular compile-time
>> configuration.
>>
>> [ Xin: build integration improvements ]
>>
>> Signed-off-by: H. Peter Anvin (Intel) <hpa@...or.com>
>> Signed-off-by: Xin Li (Intel) <xin@...or.com>
> 
> Overall LGTM, some minor points below.
> 
> 
> Reviewed-by: Nikolay Borisov <nik.borisov@...e.com>

Thanks!

> 
> 
> <snip>
> 
>> diff --git a/arch/x86/tools/featuremasks.awk 
>> b/arch/x86/tools/featuremasks.awk
>> new file mode 100755
>> index 000000000000..989b021e73d3
>> --- /dev/null
>> +++ b/arch/x86/tools/featuremasks.awk
>> @@ -0,0 +1,84 @@
>> +#!/usr/bin/awk
>> +#
>> +# Convert cpufeatures.h to a list of compile-time masks
>> +# Note: this blithly assumes that each word has at least one
>> +# feature defined in it; if not, something else is wrong!
>> +#
>> +
>> +BEGIN {
>> +    printf "#ifndef _ASM_X86_FEATUREMASKS_H\n";
>> +    printf "#define _ASM_X86_FEATUREMASKS_H\n\n";
>> +
>> +    file = 0
>> +}
>> +
>> +BEGINFILE {
>> +    switch (++file) {
>> +    case 1:            # cpufeatures.h
>> +        FPAT = "#[ \t]*[a-z]+|[A-Za-z0-9_]+|[^ \t]";
>> +        break;
>> +    case 2:            # .config
>> +        FPAT = "CONFIG_[A-Z0-9_]+|is not set|[yn]";
>> +        break;
>> +    }
>> +}
>> +
> 
> IMO this script could use a bit of high-level comments. Something like:
> 
> 
> # Create a dictionary of sorts, containing all defined feature bits
>> +file == 1 && $1 ~ /^#[ \t]*define$/ && $2 ~ /^X86_FEATURE_/ &&
>> +$3 == "(" && $5 == "*" && $7 == "+" && $9 == ")" {
>> +    nfeat = $4 * $6 + $8;
>> +    feat = $2;
>> +    sub(/^X86_FEATURE_/, "", feat);
>> +    feats[nfeat] = feat;
>> +}
> 
>> +file == 1 && $1 ~ /^#[ \t]*define$/ && $2 == "NCAPINTS" {
>> +    ncapints = strtonum($3);
>> +}
>> +
> 
> # Create a dictionary featstat[REQUIRED|DISABLED, FEATURE_NAME] = on | off
> 
>> +file == 2 && $1 ~ /^CONFIG_X86_[A-Z]*_FEATURE_/ {
>> +    on = ($2 == "y");
>> +    if (split($1, fs, "CONFIG_X86_|_FEATURE_") == 3)
>> +        featstat[fs[2], fs[3]] = on;
>> +}
>> +
>> +END {
>> +    sets[1] = "REQUIRED";
>> +    sets[2] = "DISABLED";
>> +
>> +    for (ns in sets) {
>> +        s = sets[ns];
>> +
>> +        printf "/*\n";
>> +        printf " * %s features:\n", s;
>> +        printf " *\n";
>> +        fstr = "";
>> +        for (i = 0; i < ncapints; i++) {
>> +            mask = 0;
>> +            for (j = 0; j < 32; j++) {
>> +                nfeat = i*32 + j;
>> +                feat = feats[nfeat];
>> +                if (feat) {
>> +                    st = !!featstat[s, feat];
>> +                    if (st) {
>> +                        nfstr = fstr " " feat;
>> +                        if (length(nfstr) > 72) {
>> +                            printf " *   %s\n", fstr;
>> +                            nfstr = " " feat;
>> +                        }
>> +                        fstr = nfstr;
>> +                    }
>> +                    mask += st * (2 ^ j);
> 
> nit: This expression can be changed to mask += (2 ^j) and moved inside 
> the 'if (st)' branch. Essentially only add a bit iff that status of the 
> relevant feature in the kconfig is y, which is signified by the value of 
> 'st' variable.

I will do a polish and send v4.

> 
>> +                }
>> +            }
>> +            masks[i] = mask;
>> +        }
>> +        printf " *   %s\n */\n\n", fstr;
>> +
>> +        for (i = 0; i < ncapints; i++) {
>> +            printf "#define %s_MASK%d\t0x%08x\n", s, i, masks[i];

I should append 'U' here too.

>> +        }
>> +
>> +        printf "#define %s_MASK_CHECK BUILD_BUG_ON_ZERO(NCAPINTS != 
>> %d)\n\n", s, ncapints;
>> +    }
>> +
>> +    printf "#endif /* _ASM_X86_FEATUREMASKS_H */\n";
>> +}
>