KVM: vmx/pmu: Pass-through LBR msrs when the guest LBR event is ACTIVE

In addition to DEBUGCTLMSR_LBR, any KVM trap caused by LBR msrs access
will result in a creation of guest LBR event per-vcpu.

If the guest LBR event is scheduled on with the corresponding vcpu context,
KVM will pass-through all LBR records msrs to the guest. The LBR callstack
mechanism implemented in the host could help save/restore the guest LBR
records during the event context switches, which reduces a lot of overhead
if we save/restore tens of LBR msrs (e.g. 32 LBR records entries) in the
much more frequent VMX transitions.

To avoid reclaiming LBR resources from any higher priority event on host,
KVM would always check the exist of guest LBR event and its state before
vm-entry as late as possible. A negative result would cancel the
pass-through state, and it also prevents real registers accesses and
potential data leakage. If host reclaims the LBR between two checks, the
interception state and LBR records can be safely preserved due to native
save/restore support from guest LBR event.

The KVM emits a pr_warn() when the LBR hardware is unavailable to the
guest LBR event. The administer is supposed to reminder users that the
guest result may be inaccurate if someone is using LBR to record
hypervisor on the host side.

Suggested-by: Andi Kleen <ak@linux.intel.com>
Co-developed-by: Wei Wang <wei.w.wang@intel.com>
Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Signed-off-by: Like Xu <like.xu@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Message-Id: <20210201051039.255478-7-like.xu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 194458077dcb..e2f8c3c68f02 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -658,6 +658,14 @@ static bool is_valid_passthrough_msr(u32 msr)
 	case MSR_IA32_RTIT_CR3_MATCH:
 	case MSR_IA32_RTIT_ADDR0_A ... MSR_IA32_RTIT_ADDR3_B:
 		/* PT MSRs. These are handled in pt_update_intercept_for_msr() */
+	case MSR_LBR_SELECT:
+	case MSR_LBR_TOS:
+	case MSR_LBR_INFO_0 ... MSR_LBR_INFO_0 + 31:
+	case MSR_LBR_NHM_FROM ... MSR_LBR_NHM_FROM + 31:
+	case MSR_LBR_NHM_TO ... MSR_LBR_NHM_TO + 31:
+	case MSR_LBR_CORE_FROM ... MSR_LBR_CORE_FROM + 8:
+	case MSR_LBR_CORE_TO ... MSR_LBR_CORE_TO + 8:
+		/* LBR MSRs. These are handled in vmx_update_intercept_for_lbr_msrs() */
 		return true;
 	}
 
@@ -6769,6 +6777,8 @@ reenter_guest:
 	pt_guest_enter(vmx);
 
 	atomic_switch_perf_msrs(vmx);
+	if (intel_pmu_lbr_is_enabled(vcpu))
+		vmx_passthrough_lbr_msrs(vcpu);
 
 	if (enable_preemption_timer)
 		vmx_update_hv_timer(vcpu);