perf/x86/intel: Ensure LBRs are disabled when a CPU is starting

Explicitly clear DEBUGCTL.LBR when a CPU is starting, prior to purging the
LBR MSRs themselves, as at least one system has been found to transfer
control to the kernel with LBRs enabled (it's unclear whether it's a BIOS
flaw or a CPU goof).  Because the kernel preserves the original DEBUGCTL,
even when toggling LBRs, leaving DEBUGCTL.LBR as is results in running
with LBRs enabled at all times.

Closes: https://lore.kernel.org/all/c9d8269bff69f6359731d758e3b1135dedd7cc61.camel@redhat.com
Reported-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20250131010721.470503-1-seanjc@google.com

arch/x86/events/intel/core.c

@@ -5042,8 +5042,11 @@ static void intel_pmu_cpu_starting(int cpu)
 
 	init_debug_store_on_cpu(cpu);
 	/*
-	 * Deal with CPUs that don't clear their LBRs on power-up.
+	 * Deal with CPUs that don't clear their LBRs on power-up, and that may
+	 * even boot with LBRs enabled.
 	 */
+	if (!static_cpu_has(X86_FEATURE_ARCH_LBR) && x86_pmu.lbr_nr)
+		msr_clear_bit(MSR_IA32_DEBUGCTLMSR, DEBUGCTLMSR_LBR_BIT);
 	intel_pmu_lbr_reset();
 
 	cpuc->lbr_sel = NULL;

arch/x86/include/asm/msr-index.h

@@ -395,7 +395,8 @@
 #define MSR_IA32_PASID_VALID		BIT_ULL(31)
 
 /* DEBUGCTLMSR bits (others vary by model): */
-#define DEBUGCTLMSR_LBR			(1UL <<  0) /* last branch recording */
+#define DEBUGCTLMSR_LBR_BIT		0	     /* last branch recording */
+#define DEBUGCTLMSR_LBR			(1UL <<  DEBUGCTLMSR_LBR_BIT)
 #define DEBUGCTLMSR_BTF_SHIFT		1
 #define DEBUGCTLMSR_BTF			(1UL <<  1) /* single-step on branches */
 #define DEBUGCTLMSR_BUS_LOCK_DETECT	(1UL <<  2)