From mboxrd@z Thu Jan  1 00:00:00 1970
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: KVM: x86: update masterclock on TSC writes
Date: Tue, 04 Nov 2014 18:10:34 +0100
Message-ID: <5459088A.50801@redhat.com>
References: <20141103211627.GA19724@amt.cnet>
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Cc: kvm-devel <kvm@vger.kernel.org>
To: Marcelo Tosatti <mtosatti@redhat.com>
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Extending the context we have:
>         if (vcpus_matched && gtod->clock.vclock_mode == VCLOCK_TSC)
>  		if (!ka->use_master_clock)
>  			do_request = 1;
>  
> -	if (!vcpus_matched && ka->use_master_clock)
> +	if (ka->use_master_clock)
>  			do_request = 1;
>  
>         if (do_request)
>                 kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);

The patch also makes the previous "if (!ka->use_master_clock)" redundant.
If you enter the first "if", do_request will be 1 independent of
ka->use_master_clock.  So you should also drop that one, and possibly
rewrite it simply like this:

	if ((vcpus_matched && gtod->clock.vclock_mode == VCLOCK_TSC) ||
	    ka->use_master_clock)
		kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);

But this brings the question: what is vclock_mode in your case?  If it
is VCLOCK_TSC, are you sure that the bug is fixed because you modified
the second "if", or could it be fixed also by removing instead the
"if (!ka->use_master_clock)"?  This would leave the optimization in the
case "!vcpus_matched && ka->use_master_clock".  Or is the optimization
always invalid?

A different way to state the same question: can you explain the
resulting condition

	((vcpus_matched && gtod->clock.vclock_mode == VCLOCK_TSC) ||
	 ka->use_master_clock)

?  Please add a comment to kvm_track_tsc_matching that clarifies this
logic.

Paolo