perf/x86/intel/lbr: Fix unchecked MSR access error on HSW

The fuzzer triggers the below trace.

[ 7763.384369] unchecked MSR access error: WRMSR to 0x689
(tried to write 0x1fffffff8101349e) at rIP: 0xffffffff810704a4
(native_write_msr+0x4/0x20)
[ 7763.397420] Call Trace:
[ 7763.399881]  <TASK>
[ 7763.401994]  intel_pmu_lbr_restore+0x9a/0x1f0
[ 7763.406363]  intel_pmu_lbr_sched_task+0x91/0x1c0
[ 7763.410992]  __perf_event_task_sched_in+0x1cd/0x240

On a machine with the LBR format LBR_FORMAT_EIP_FLAGS2, when the TSX is
disabled, a TSX quirk is required to access LBR from registers.
The lbr_from_signext_quirk_needed() is introduced to determine whether
the TSX quirk should be applied. However, the
lbr_from_signext_quirk_needed() is invoked before the
intel_pmu_lbr_init(), which parses the LBR format information. Without
the correct LBR format information, the TSX quirk never be applied.

Move the lbr_from_signext_quirk_needed() into the intel_pmu_lbr_init().
Checking x86_pmu.lbr_has_tsx in the lbr_from_signext_quirk_needed() is
not required anymore.

Both LBR_FORMAT_EIP_FLAGS2 and LBR_FORMAT_INFO have LBR_TSX flag, but
only the LBR_FORMAT_EIP_FLAGS2 requirs the quirk. Update the comments
accordingly.

Fixes: 1ac7fd8 ("perf/x86/intel/lbr: Support LBR format V7")
Reported-by: Vince Weaver <[email protected]>
Signed-off-by: Kan Liang <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]

Author: Kan Liang

arch/x86/events/intel/lbr.c

@@ -278,17 +278,17 @@ enum {
 };
 
 /*
- * For formats with LBR_TSX flags (e.g. LBR_FORMAT_EIP_FLAGS2), bits 61:62 in
- * MSR_LAST_BRANCH_FROM_x are the TSX flags when TSX is supported, but when
- * TSX is not supported they have no consistent behavior:
+ * For format LBR_FORMAT_EIP_FLAGS2, bits 61:62 in MSR_LAST_BRANCH_FROM_x
+ * are the TSX flags when TSX is supported, but when TSX is not supported
+ * they have no consistent behavior:
  *
  *   - For wrmsr(), bits 61:62 are considered part of the sign extension.
  *   - For HW updates (branch captures) bits 61:62 are always OFF and are not
  *     part of the sign extension.
  *
  * Therefore, if:
  *
- *   1) LBR has TSX format
+ *   1) LBR format LBR_FORMAT_EIP_FLAGS2
  *   2) CPU has no TSX support enabled
  *
  * ... then any value passed to wrmsr() must be sign extended to 63 bits and any
@@ -300,7 +300,7 @@ static inline bool lbr_from_signext_quirk_needed(void)
 	bool tsx_support = boot_cpu_has(X86_FEATURE_HLE) ||
 			   boot_cpu_has(X86_FEATURE_RTM);
 
-	return !tsx_support && x86_pmu.lbr_has_tsx;
+	return !tsx_support;
 }
 
 static DEFINE_STATIC_KEY_FALSE(lbr_from_quirk_key);
@@ -1609,9 +1609,6 @@ void intel_pmu_lbr_init_hsw(void)
 	x86_pmu.lbr_sel_map  = hsw_lbr_sel_map;
 
 	x86_get_pmu(smp_processor_id())->task_ctx_cache = create_lbr_kmem_cache(size, 0);
-
-	if (lbr_from_signext_quirk_needed())
-		static_branch_enable(&lbr_from_quirk_key);
 }
 
 /* skylake */
@@ -1702,7 +1699,11 @@ void intel_pmu_lbr_init(void)
 	switch (x86_pmu.intel_cap.lbr_format) {
 	case LBR_FORMAT_EIP_FLAGS2:
 		x86_pmu.lbr_has_tsx = 1;
-		fallthrough;
+		x86_pmu.lbr_from_flags = 1;
+		if (lbr_from_signext_quirk_needed())
+			static_branch_enable(&lbr_from_quirk_key);
+		break;
+
 	case LBR_FORMAT_EIP_FLAGS:
 		x86_pmu.lbr_from_flags = 1;
 		break;