On 2011-04-14 00:27, Jung-uk Kim wrote:
...
will still read 0 from MSR_MPERF, leading to a division by zero.
Maybe just fallback to the second method in the 'else' branch then?
That means your VM has broken CPUID support. To get there, it has to
meet two conditions, i.e., TSC is invariant and it has APERF/MPERF
MSRs.
Well, VM hosts like VMware and VirtualBox usually just return the
'native' CPUID values to guests, but can't really support stuff like
those MSRs, for all kinds of reasons.
I was just looking at this from a viewpoint of "it worked for years, and
now it broke". :)
In any case, I don't see why a bit of defensive programming would be bad
here, so I propose the following patch to revert to the 'old' way of
estimating the rate, in case reading the MPERF MSR returns zero.
Index: sys/i386/i386/machdep.c
===================================================================
--- sys/i386/i386/machdep.c (revision 220620)
+++ sys/i386/i386/machdep.c (working copy)
@@ -1172,7 +1172,7 @@ cpu_est_clockrate(int cpu_id, uint64_t *rate)
acnt = rdmsr(MSR_APERF);
tsc2 = rdtsc();
intr_restore(reg);
- perf = 1000 * acnt / mcnt;
+ perf = (mcnt != 0) ? 1000 * acnt / mcnt : 1000;
*rate = (tsc2 - tsc1) * perf;
} else {
tsc1 = rdtsc();
Index: sys/amd64/amd64/machdep.c
===================================================================
--- sys/amd64/amd64/machdep.c (revision 220620)
+++ sys/amd64/amd64/machdep.c (working copy)
@@ -579,7 +579,7 @@ cpu_est_clockrate(int cpu_id, uint64_t *rate)
acnt = rdmsr(MSR_APERF);
tsc2 = rdtsc();
intr_restore(reg);
- perf = 1000 * acnt / mcnt;
+ perf = (mcnt != 0) ? 1000 * acnt / mcnt : 1000;
*rate = (tsc2 - tsc1) * perf;
} else {
tsc1 = rdtsc();
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