svn commit: r220584 - in head/sys: amd64/amd64 i386/i386

Dimitry Andric dim at FreeBSD.org
Thu Apr 14 10:49:22 UTC 2011


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.
-------------- next part --------------
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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