[PATCH 2/4] perf/x86/intel: Don't context switch DS_AREA (and PEBS config) if PEBS is unused

[Sean Christopherson]

When filling the list of MSRs to be loaded by KVM on VM-Enter and VM-Exit,
insert DS_AREA and (conditionally) MSR_PEBS_DATA_CFG into the list if and
only if PEBS will be active in the guest, i.e. only if a PEBS record may be
generated while running the guest.  As shown by the !x86_pmu.pebs_ept path,
it's perfectly safe to run with the host's DS_AREA, so long as PEBS-enabled
counters are disabled via PERF_GLOBAL_CTRL.

Omitting DS_AREA and MSR_PEBS_DATA_CFG when PEBS is unused saves two MSR
writes per MSR on each VMX transition, i.e. eliminates two/four pointless
MSR writes on each VMX roundtrip when PEBS isn't being used by the guest.

Fixes: c59a1f106f5c ("KVM: x86/pmu: Add IA32_PEBS_ENABLE MSR emulation for extended PEBS")
Cc: Jim Mattson <jmattson@xxxxxxxxxx>
Cc: Mingwei Zhang <mizhang@xxxxxxxxxx>
Cc: Stephane Eranian <eranian@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/events/intel/core.c | 41 ++++++++++++++++++++++++------------
 1 file changed, 27 insertions(+), 14 deletions(-)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index 002d809f82ef..20a153aa33cb 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -5037,23 +5037,14 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data)
 		return arr;
 	}
 
+	/*
+	 * If the guest won't use PEBS or the CPU doesn't support PEBS in the
+	 * guest, then there's nothing more to do as disabling PMCs via
+	 * PERF_GLOBAL_CTRL is sufficient on CPUs with guest/host isolation.
+	 */
 	if (!kvm_pmu || !x86_pmu.pebs_ept)
 		return arr;
 
-	arr[(*nr)++] = (struct perf_guest_switch_msr){
-		.msr = MSR_IA32_DS_AREA,
-		.host = (unsigned long)cpuc->ds,
-		.guest = kvm_pmu->ds_area,
-	};
-
-	if (x86_pmu.intel_cap.pebs_baseline) {
-		arr[(*nr)++] = (struct perf_guest_switch_msr){
-			.msr = MSR_PEBS_DATA_CFG,
-			.host = cpuc->active_pebs_data_cfg,
-			.guest = kvm_pmu->pebs_data_cfg,
-		};
-	}
-
 	/*
 	 * Disable counters where the guest PMC is different than the host PMC
 	 * being used on behalf of the guest, as the PEBS record includes
@@ -5065,6 +5056,28 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data)
 	if (pebs_mask & ~cpuc->intel_ctrl_guest_mask)
 		guest_pebs_mask = 0;
 
+	/*
+	 * Context switch DS_AREA and PEBS_DATA_CFG if and only if PEBS will be
+	 * active in the guest; if no records will be generated while the guest
+	 * is running, then running with host values is safe (see above).
+	 */
+	if (!guest_pebs_mask)
+		return arr;
+
+	arr[(*nr)++] = (struct perf_guest_switch_msr){
+		.msr = MSR_IA32_DS_AREA,
+		.host = (unsigned long)cpuc->ds,
+		.guest = kvm_pmu->ds_area,
+	};
+
+	if (x86_pmu.intel_cap.pebs_baseline) {
+		arr[(*nr)++] = (struct perf_guest_switch_msr){
+			.msr = MSR_PEBS_DATA_CFG,
+			.host = cpuc->active_pebs_data_cfg,
+			.guest = kvm_pmu->pebs_data_cfg,
+		};
+	}
+
 	/*
 	 * Do NOT mess with PEBS_ENABLED.  As above, disabling counters via
 	 * PERF_GLOBAL_CTRL is sufficient, and loading a stale PEBS_ENABLED,
-- 
2.54.0.rc0.605.g598a273b03-goog