KVM: x86: Fix perfctr WRMSR for running counters

Correct the logic in intel_pmu_set_msr() for fixed and general purpose
counters. This was recently changed to set pmc->counter without taking
in to account the value of pmc_read_counter() which will be incorrect if
the counter is currently running and non-zero; this changes back to the
old logic which accounted for the value of currently running counters.
Signed-off-by: Eric Hankland <ehankland@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/pmu_intel.c

@@ -260,13 +260,12 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		break;
 	default:
 		if ((pmc = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0))) {
-			if (msr_info->host_initiated)
-				pmc->counter = data;
-			else
-				pmc->counter = (s32)data;
+			if (!msr_info->host_initiated)
+				data = (s64)(s32)data;
+			pmc->counter += data - pmc_read_counter(pmc);
 			return 0;
 		} else if ((pmc = get_fixed_pmc(pmu, msr))) {
-			pmc->counter = data;
+			pmc->counter += data - pmc_read_counter(pmc);
 			return 0;
 		} else if ((pmc = get_gp_pmc(pmu, msr, MSR_P6_EVNTSEL0))) {
 			if (data == pmc->eventsel)