port-amd64/47967: DTrace does not work while running under QEMU

[riz%NetBSD.org@localhost]

>Number:         47967
>Category:       port-amd64
>Synopsis:       tsc_freq is 0 when running under QEMU so DTrace crashes at load
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    port-amd64-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Jun 26 17:35:00 +0000 2013
>Originator:     Jeff Rizzo
>Release:        NetBSD 6.99.22
>Organization:
        
>Environment:
        
        
System: NetBSD slash.lan 6.99.22 NetBSD 6.99.22 (DTRACE) #37: Sat Jun 22 
16:37:43 PDT 2013 
riz%slash.lan@localhost:/build/obj/sys/arch/amd64/compile/DTRACE amd64
Architecture: x86_64
Machine: amd64
>Description:
        QEMU does not implement MSR_TSC, so reads there with rdmsr() always
        return 0, which breaks cpu_counter_serializing(), and in turn
        breaks DTrace under QEMU.

        Things work ok if cpu_counter_serializing() is made to use
        cpu_counter() when running under QEMU.

>How-To-Repeat:
        Install system built with MKDTRACE=yes under QEMU, and
        try to "modload dtrace".  Boom.
>Fix:

This patch makes things work for me, though I'm not convinced it's
entirely correct:

Index: sys/arch/x86/x86/identcpu.c
===================================================================
RCS file: /cvsroot/src/sys/arch/x86/x86/identcpu.c,v
retrieving revision 1.32
diff -u -r1.32 identcpu.c
--- sys/arch/x86/x86/identcpu.c 16 Jun 2012 17:30:19 -0000      1.32
+++ sys/arch/x86/x86/identcpu.c 26 Jun 2013 16:08:01 -0000
@@ -563,6 +563,17 @@
 }

 static void
+cpu_probe_qemu(struct cpu_info *ci)
+{
+       if (memcmp("QEMU Virtual CPU", cpu_brand_string, 16) != 0)
+               return;
+
+       /* if QEMU does not implement MSR_TSC, disable the TSC */
+       ci->ci_feat_val[0] &= ~CPUID_MSR;
+       cpu_feature[0] &= ~CPUID_MSR;
+}
+
+static void
 cpu_probe_vortex86(struct cpu_info *ci)
 {
 #define PCI_MODE1_ADDRESS_REG  0x0cf8
@@ -709,6 +720,7 @@
        cpu_probe_c3(ci);
        cpu_probe_geode(ci);
        cpu_probe_vortex86(ci);
+       cpu_probe_qemu(ci);

        x86_cpu_topology(ci);

>Unformatted: