Re: [PATCH]: Fix MSR_K7_EVNTSEL{0,3} for SVM

[Chris Lalancette <clalance@redhat.com>]

Avi Kivity wrote:
> 
> Hmm, looking back at the dump:
> 
>>     1811:	8d 86 00 00 ff 3f    	lea    0x3fff0000(%rsi),%eax
>>     1817:	83 f8 03             	cmp    $0x3,%eax
>>     181a:	0f 87 e2 01 00 00    	ja     1a02 <svm_set_msr+0x27f>
> 
> So while gcc is using %rsi, it loads the result back into %eax, which 
> has the effect of dropping back into 32-bits.  So looks like gcc was 
> right here.  Sorry for spreading confusion and apologies to gcc.
> 

Avi,
     Arg.  I was completely, utterly wrong about the problem here (although
there is definitely still a problem).  I'm sorry for making a confusing mess out
of this.  Here is what is actually happening:

During startup, the RHEL-4 x86_64 kernel (2.6.9-67.EL, if you care) setups up
the NMI watchdog.  It does the following:

	for(i = 0; i < 4; ++i) {
		/* Simulator may not support it */
		if (checking_wrmsrl(MSR_K7_EVNTSEL0+i, 0UL))
			return;
		wrmsrl(MSR_K7_PERFCTR0+i, 0UL);
	}

checking_wrmsrl() just does a "test write" to the msr; because of the code that
is currently in there, this succeeds.  However, when it tries to do the
MSR_K7_PERFCTR0 wrmsr, *that* is where it fails, since we don't currently handle
that MSR, and KVM injects a GPF into the guest (which kills it).  My previous
patch just happened to fix this because it was making checking_wrmsrl() fail on
the EVNTSEL0, so we just returned out of this loop rather than trying to write
to the PERFCTR0.

Unfortunately, we can't just "fake emulate" MSR_K7_PERFCTR[0-3] like we are
doing for MSR_K7_EVNTSEL[0-3]; if they are there, linux expects to be able to
put values into them.  I think the correct solution here is to emulate
MSR_K7_PERFCTR[0-3] and MSR_K7_EVNTSEL[0-3] for real.  I'm working on a patch to
do this now.

Chris Lalancette

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