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

commit c631a2de7ae48d50434bdc205d901423f8577c65 upstream.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index b70e1522a27ac6c4992baf141510d53a02e49081..767c60af13be33da92759f243bd98e744481267f 100644 (file)
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -4364,8 +4364,11 @@ 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;

diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 15939a71dca7c75c52df497d89d62ca095b88da0..03b12c19458892d0c03cf814c85890d83fd55597 100644 (file)
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -337,7 +337,8 @@
 #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)