[PATCH] target/ppc/kvm: Cache timebase frequency

[Greg Kurz]

Each vCPU core exposes its timebase frequency in the DT. When running
under KVM, this means parsing /proc/cpuinfo in order to get the timebase
frequency of the host CPU.

The parsing appears to slow down the boot quite a bit with higher number
of cores:

# of cores     seconds spent in spapr_dt_cpus()
      8                  0.550122
     16                  1.342375
     32                  2.850316
     64                  5.922505
     96                  9.109224
    128                 12.245504
    256                 24.957236
    384                 37.389113

The timebase frequency of the host CPU is identical for all
cores and it is an invariant for the VM lifetime. Cache it
instead of doing the same expensive parsing again and again.

With this patch applied:

    384                 0.518382

Signed-off-by: Greg Kurz <groug@kaod.org>
---
 target/ppc/kvm.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
index 298c1f882c67..9ad3dae29132 100644
--- a/target/ppc/kvm.c
+++ b/target/ppc/kvm.c
@@ -1819,7 +1819,13 @@ uint32_t kvmppc_get_tbfreq(void)
 {
     char line[512];
     char *ns;
-    uint32_t retval = NANOSECONDS_PER_SECOND;
+    static uint32_t retval = -1;
+
+    if (retval != -1) {
+        return retval;
+    }
+
+    retval = NANOSECONDS_PER_SECOND;
 
     if (read_cpuinfo("timebase", line, sizeof(line))) {
         return retval;
@@ -1832,7 +1838,8 @@ uint32_t kvmppc_get_tbfreq(void)
 
     ns++;
 
-    return atoi(ns);
+    retval = atoi(ns);
+    return retval;
 }
 
 bool kvmppc_get_host_serial(char **value)