cpu_counter_serializing() vs. QEMU

[Jeff Rizzo <riz%tastylime.net@localhost>]

This all came about because I discovered that "modload dtrace" while 
running under QEMU (amd64, but I bet the same thing happens under i386) 
panics the kernel, because tsc_freq is 0.  Hm.

Digging a little deeper, in sys/arch/x86/x86/tsc.c:

uint64_t
cpu_counter_serializing(void)
{
        if (cpu_feature[0] & CPUID_MSR)
                return rdmsr(MSR_TSC);
        else
                return cpu_counter();
}


Under QEMU, rdmsr(MSR_TSC) always returns 0, because that MSR isn't 
implemented (apparently).  If I hardcode cpu_counter_serializing() to 
always return cpu_counter(), things work just fine.

A couple things:

- is testing (cpu_feature[0] & CPUID_MSR) the right test?  Only some 
MSRs are implemented.

- if that test is correct, what's the right way/best place to check if 
rdmsr(MSR_TSC) returns 0, and hence use cpu_counter() instead?

I'd be surprised if dtrace is the only thing suffering because of this...

+j