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

From: Nikolay Borisov
Date: Wed Jun 26 2024 - 07:36:27 EST




On 22.06.24 г. 20:14 ч., 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

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@xxxxxxxxx>
Signed-off-by: Xin Li (Intel) <xin@xxxxxxxxx>

Overall LGTM, some minor points below.


Reviewed-by: Nikolay Borisov <nik.borisov@xxxxxxxx>


<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.

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