cpufreq: intel_pstate: Check IDA only before MSR_IA32_PERF_CTL writes

JIRA: https://issues.redhat.com/browse/RHEL-128549

commit 4b747cc
Author: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Date:   Mon Nov 10 17:08:40 2025 -0800

    cpufreq: intel_pstate: Check IDA only before MSR_IA32_PERF_CTL writes

    Commit ac4e04d ("cpufreq: intel_pstate: Unchecked MSR aceess in
    legacy mode") introduced a check for feature X86_FEATURE_IDA to verify
    turbo mode support. Although this is the correct way to check for turbo
    mode support, it causes issues on some platforms that disable turbo
    during OS boot, but enable it later [1]. Before adding this feature
    check, users were able to get turbo mode frequencies by writing 0 to
    /sys/devices/system/cpu/intel_pstate/no_turbo post-boot.

    To restore the old behavior on the affected systems while still
    addressing the unchecked MSR issue on some Skylake-X systems, check
    X86_FEATURE_IDA only immediately before updates of MSR_IA32_PERF_CTL
    that may involve setting the Turbo Engage Bit (bit 32).

    Fixes: ac4e04d ("cpufreq: intel_pstate: Unchecked MSR aceess in legacy mode")
    Reported-by: Aaron Rainbolt <arainbolt@kfocus.org>
    Closes: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2122531 [1]
    Tested-by: Aaron Rainbolt <arainbolt@kfocus.org>
    Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
    [ rjw: Subject adjustment, changelog edits ]
    Link: https://patch.msgid.link/20251111010840.141490-1-srinivas.pandruvada@linux.intel.com
    Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

(cherry picked from commit 4b747cc)
Assisted-by: Patchpal
Signed-off-by: David Arcari <darcari@redhat.com>

Author: darcari

drivers/cpufreq/intel_pstate.c

@@ -604,9 +604,6 @@ static bool turbo_is_disabled(void)
 {
 	u64 misc_en;
 
-	if (!cpu_feature_enabled(X86_FEATURE_IDA))
-		return true;
-
 	rdmsrl(MSR_IA32_MISC_ENABLE, misc_en);
 
 	return !!(misc_en & MSR_IA32_MISC_ENABLE_TURBO_DISABLE);
@@ -2151,7 +2148,8 @@ static u64 atom_get_val(struct cpudata *cpudata, int pstate)
 	u32 vid;
 
 	val = (u64)pstate << 8;
-	if (READ_ONCE(global.no_turbo) && !READ_ONCE(global.turbo_disabled))
+	if (READ_ONCE(global.no_turbo) && !READ_ONCE(global.turbo_disabled) &&
+	    cpu_feature_enabled(X86_FEATURE_IDA))
 		val |= (u64)1 << 32;
 
 	vid_fp = cpudata->vid.min + mul_fp(
@@ -2316,7 +2314,8 @@ static u64 core_get_val(struct cpudata *cpudata, int pstate)
 	u64 val;
 
 	val = (u64)pstate << 8;
-	if (READ_ONCE(global.no_turbo) && !READ_ONCE(global.turbo_disabled))
+	if (READ_ONCE(global.no_turbo) && !READ_ONCE(global.turbo_disabled) &&
+	    cpu_feature_enabled(X86_FEATURE_IDA))
 		val |= (u64)1 << 32;
 
 	return val;