x86/cpu: Log CPU flag cmdline hacks more verbosely

Since using these options is very dangerous, make details as visible as
possible:

- Instead of a single message for each of the cmdline options, print a
  separate pr_warn() for each individual flag.

- Say explicitly whether the flag is a "feature" or a "bug".

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20250303-setcpuid-taint-louder-v1-3-8d255032cb4c@google.com

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index c1ced31f976d970efd24d6c9e4ac77cbff3371b9..8eba9ca9c216127ce1a6d630e7f67130520137b4 100644 (file)
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1484,8 +1484,6 @@ static inline bool parse_set_clear_cpuid(char *arg, bool set)
        char *opt;
        int taint = 0;
 
-       pr_warn("%s CPUID bits:", set ? "Force-enabling" : "Clearing");
-
        while (arg) {
                bool found __maybe_unused = false;
                unsigned int bit;
@@ -1500,16 +1498,19 @@ static inline bool parse_set_clear_cpuid(char *arg, bool set)
                if (!kstrtouint(opt, 10, &bit)) {
                        if (bit < NCAPINTS * 32) {
 
+                               if (set) {
+                                       pr_warn("setcpuid: force-enabling CPU feature flag:");
+                                       setup_force_cpu_cap(bit);
+                               } else {
+                                       pr_warn("clearcpuid: force-disabling CPU feature flag:");
+                                       setup_clear_cpu_cap(bit);
+                               }
                                /* empty-string, i.e., ""-defined feature flags */
                                if (!x86_cap_flags[bit])
-                                       pr_cont(" %d:%d", bit >> 5, bit & 31);
+                                       pr_cont(" %d:%d\n", bit >> 5, bit & 31);
                                else
-                                       pr_cont(" %s", x86_cap_flags[bit]);
+                                       pr_cont(" %s\n", x86_cap_flags[bit]);
 
-                               if (set)
-                                       setup_force_cpu_cap(bit);
-                               else
-                                       setup_clear_cpu_cap(bit);
                                taint++;
                        }
                        /*
@@ -1521,11 +1522,15 @@ static inline bool parse_set_clear_cpuid(char *arg, bool set)
 
                for (bit = 0; bit < 32 * (NCAPINTS + NBUGINTS); bit++) {
                        const char *flag;
+                       const char *kind;
 
-                       if (bit < 32 * NCAPINTS)
+                       if (bit < 32 * NCAPINTS) {
                                flag = x86_cap_flags[bit];
-                       else
+                               kind = "feature";
+                       } else {
+                               kind = "bug";
                                flag = x86_bug_flags[bit - (32 * NCAPINTS)];
+                       }
 
                        if (!flag)
                                continue;
@@ -1533,22 +1538,24 @@ static inline bool parse_set_clear_cpuid(char *arg, bool set)
                        if (strcmp(flag, opt))
                                continue;
 
-                       pr_cont(" %s", opt);
-                       if (set)
+                       if (set) {
+                               pr_warn("setcpuid: force-enabling CPU %s flag: %s\n",
+                                       kind, flag);
                                setup_force_cpu_cap(bit);
-                       else
+                       } else {
+                               pr_warn("clearcpuid: force-disabling CPU %s flag: %s\n",
+                                       kind, flag);
                                setup_clear_cpu_cap(bit);
+                       }
                        taint++;
                        found = true;
                        break;
                }
 
                if (!found)
-                       pr_cont(" (unknown: %s)", opt);
+                       pr_warn("%s: unknown CPU flag: %s", set ? "setcpuid" : "clearcpuid", opt);
        }
 
-       pr_cont("\n");
-
        return taint;
 }