KVM: vmx/pmu: Expose LBR_FMT in the MSR_IA32_PERF_CAPABILITIES

Userspace could enable guest LBR feature when the exactly supported
LBR format value is initialized to the MSR_IA32_PERF_CAPABILITIES
and the LBR is also compatible with vPMU version and host cpu model.

The LBR could be enabled on the guest if host perf supports LBR
(checked via x86_perf_get_lbr()) and the vcpu model is compatible
with the host one.
Signed-off-by: Like Xu <like.xu@linux.intel.com>
Message-Id: <20210201051039.255478-11-like.xu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/capabilities.h

@@ -380,11 +380,18 @@ static inline bool vmx_pt_mode_is_host_guest(void)
 
 static inline u64 vmx_get_perf_capabilities(void)
 {
+	u64 perf_cap = 0;
+
+	if (boot_cpu_has(X86_FEATURE_PDCM))
+		rdmsrl(MSR_IA32_PERF_CAPABILITIES, perf_cap);
+
+	perf_cap &= PMU_CAP_LBR_FMT;
+
 	/*
 	 * Since counters are virtualized, KVM would support full
 	 * width counting unconditionally, even if the host lacks it.
 	 */
-	return PMU_CAP_FW_WRITES;
+	return PMU_CAP_FW_WRITES | perf_cap;
 }
 
 static inline u64 vmx_supported_debugctl(void)