kvm: Set the perf exclude_host attribute if available

The performance counting framework in Linux 3.2 and onwards supports
an attribute to exclude events generated by the host when running
KVM. Setting this attribute allows us to get more reliable
measurements of the guest machine. For example, on a highly loaded
system, the instruction counts from the guest can be severely
distorted by the host kernel (e.g., by page fault handlers).

This changeset introduces a check for the attribute and enables it in
the KVM CPU if present.

1 files changed, 12 insertions, 0 deletions

diff --git a/src/cpu/kvm/base.cc b/src/cpu/kvm/base.cc
index a8e115da7..d25e145a5 100644
--- a/src/cpu/kvm/base.cc
+++ b/src/cpu/kvm/base.cc
@@ -1133,6 +1133,12 @@ BaseKvmCPU::setupCounters()
     cfgCycles.disabled(true)
         .pinned(true);
 
+    // Try to exclude the host. We set both exclude_hv and
+    // exclude_host since different architectures use slightly
+    // different APIs in the kernel.
+    cfgCycles.exclude_hv(true)
+        .exclude_host(true);
+
     if (perfControlledByTimer) {
         // We need to configure the cycles counter to send overflows
         // since we are going to use it to trigger timer signals that
@@ -1206,6 +1212,12 @@ BaseKvmCPU::setupInstCounter(uint64_t period)
     PerfKvmCounterConfig cfgInstructions(PERF_TYPE_HARDWARE,
                                          PERF_COUNT_HW_INSTRUCTIONS);
 
+    // Try to exclude the host. We set both exclude_hv and
+    // exclude_host since different architectures use slightly
+    // different APIs in the kernel.
+    cfgInstructions.exclude_hv(true)
+        .exclude_host(true);
+
     if (period) {
         // Setup a sampling counter if that has been requested.
         cfgInstructions.wakeupEvents(1)