Re: [PATCH] KVM: SVM: handle singlestep exception when skipping emulated instructions

["Radim Krčmář" <rkrcmar@redhat.com>]

2017-06-21 18:55+0200, Radim Krčmář:
> 2017-06-21 15:37+0200, Ladi Prosek:
> > kvm_skip_emulated_instruction handles the singlestep debug exception
> > which is something we almost always want. This commit (specifically
> > the change in rdmsr_interception) makes the debug.flat KVM unit test
> > pass on AMD.
> 
> kvm_skip_emulated_instruction() also has a return value, which says
> whether the debug exception was requested by the userspace or by the
> guest (userspace has priority).
> 
> This patch fixes the guest debugging, but userspace still won't receive
> its events.  I think it would be better to fix both at once,
> 
> > Signed-off-by: Ladi Prosek <lprosek@redhat.com>
> > ---
> > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> > @@ -2278,7 +2278,7 @@ static int io_interception(struct vcpu_svm *svm)
> >  	port = io_info >> 16;
> >  	size = (io_info & SVM_IOIO_SIZE_MASK) >> SVM_IOIO_SIZE_SHIFT;
> >  	svm->next_rip = svm->vmcb->control.exit_info_2;
> > -	skip_emulated_instruction(&svm->vcpu);
> > +	kvm_skip_emulated_instruction(&svm->vcpu);
> >  
> >  	return in ? kvm_fast_pio_in(vcpu, size, port)
> >  		  : kvm_fast_pio_out(vcpu, size, port);
> 
> i.e.
> 	ret = kvm_skip_emulated_instruction(&svm->vcpu);
> 
> 	return ret && (...);

Nope, the ret has to be checked afterwards ... better look at
handle_io() in vmx.c. :)