perf/x86/core: Zero @lbr instead of returning -1 in x86_perf_get_lbr() stub

Drop the return value from x86_perf_get_lbr() and have the stub zero out
the @lbr structure instead of returning -1 to indicate "no LBR support".
KVM doesn't actually check the return value, and instead subtly relies on
zeroing the number of LBRs in intel_pmu_init().

Formalize "nr=0 means unsupported" so that KVM doesn't need to add a
pointless check on the return value to fix KVM's benign bug.

Note, the stub is necessary even though KVM x86 selects PERF_EVENTS and
the caller exists only when CONFIG_KVM_INTEL=y.  Despite the name,
KVM_INTEL doesn't strictly require CPU_SUP_INTEL, it can be built with
any of INTEL || CENTAUR || ZHAOXIN CPUs.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221006000314.73240-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
index 8259d72..4dbde69 100644 (file)
--- a/arch/x86/events/intel/lbr.c
+++ b/arch/x86/events/intel/lbr.c
@@ -1603,10 +1603,8 @@ clear_arch_lbr:
  * x86_perf_get_lbr - get the LBR records information
  *
  * @lbr: the caller's memory to store the LBR records information
- *
- * Returns: 0 indicates the LBR info has been successfully obtained
  */
-int x86_perf_get_lbr(struct x86_pmu_lbr *lbr)
+void x86_perf_get_lbr(struct x86_pmu_lbr *lbr)
 {
        int lbr_fmt = x86_pmu.intel_cap.lbr_format;
 
@@ -1614,8 +1612,6 @@ int x86_perf_get_lbr(struct x86_pmu_lbr *lbr)
        lbr->from = x86_pmu.lbr_from;
        lbr->to = x86_pmu.lbr_to;
        lbr->info = (lbr_fmt == LBR_FORMAT_INFO) ? x86_pmu.lbr_info : 0;
-
-       return 0;
 }
 EXPORT_SYMBOL_GPL(x86_perf_get_lbr);
 

diff --git a/arch/x86/include/asm/perf_event.h b/arch/x86/include/asm/perf_event.h
index 9ac46db..5d0f689 100644 (file)
--- a/arch/x86/include/asm/perf_event.h
+++ b/arch/x86/include/asm/perf_event.h
@@ -543,12 +543,12 @@ static inline void perf_check_microcode(void) { }
 
 #if defined(CONFIG_PERF_EVENTS) && defined(CONFIG_CPU_SUP_INTEL)
 extern struct perf_guest_switch_msr *perf_guest_get_msrs(int *nr, void *data);
-extern int x86_perf_get_lbr(struct x86_pmu_lbr *lbr);
+extern void x86_perf_get_lbr(struct x86_pmu_lbr *lbr);
 #else
 struct perf_guest_switch_msr *perf_guest_get_msrs(int *nr, void *data);
-static inline int x86_perf_get_lbr(struct x86_pmu_lbr *lbr)
+static inline void x86_perf_get_lbr(struct x86_pmu_lbr *lbr)
 {
-       return -1;
+       memset(lbr, 0, sizeof(*lbr));
 }
 #endif
 

diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
index 0725431..479124e 100644 (file)
--- a/arch/x86/kvm/vmx/capabilities.h
+++ b/arch/x86/kvm/vmx/capabilities.h
@@ -407,7 +407,8 @@ static inline u64 vmx_get_perf_capabilities(void)
        if (boot_cpu_has(X86_FEATURE_PDCM))
                rdmsrl(MSR_IA32_PERF_CAPABILITIES, host_perf_cap);
 
-       if (x86_perf_get_lbr(&lbr) >= 0 && lbr.nr)
+       x86_perf_get_lbr(&lbr);
+       if (lbr.nr)
                perf_cap |= host_perf_cap & PMU_CAP_LBR_FMT;
 
        if (vmx_pebs_supported()) {