Re: [PATCH 1/6] KVM: PPC: BOOKE: No need to set DBCR0_EDM in guest visible register

[Scott Wood <scottwood@freescale.com>]

On Wed, 2014-07-30 at 12:57 -0500, Bhushan Bharat-R65777 wrote:
> 
> > -----Original Message-----
> > From: Wood Scott-B07421
> > Sent: Wednesday, July 30, 2014 11:18 PM
> > To: Bhushan Bharat-R65777
> > Cc: agraf@suse.de; kvm-ppc@vger.kernel.org; kvm@vger.kernel.org; Yoder Stuart-
> > B08248
> > Subject: Re: [PATCH 1/6] KVM: PPC: BOOKE: No need to set DBCR0_EDM in guest
> > visible register
> > 
> > On Wed, 2014-07-30 at 00:21 -0500, Bhushan Bharat-R65777 wrote:
> > >
> > > > -----Original Message-----
> > > > From: Wood Scott-B07421
> > > > Sent: Tuesday, July 29, 2014 3:22 AM
> > > > To: Bhushan Bharat-R65777
> > > > Cc: agraf@suse.de; kvm-ppc@vger.kernel.org; kvm@vger.kernel.org;
> > > > Yoder Stuart-
> > > > B08248
> > > > Subject: Re: [PATCH 1/6] KVM: PPC: BOOKE: No need to set DBCR0_EDM
> > > > in guest visible register
> > > >
> > > > On Fri, 2014-07-11 at 14:08 +0530, Bharat Bhushan wrote:
> > > > > This is not used and  even I do not remember why this was added in
> > > > > first place.
> > > > >
> > > > > Signed-off-by: Bharat Bhushan <Bharat.Bhushan@freescale.com>
> > > > > ---
> > > > >  arch/powerpc/kvm/booke.c | 2 --
> > > > >  1 file changed, 2 deletions(-)
> > > > >
> > > > > diff --git a/arch/powerpc/kvm/booke.c b/arch/powerpc/kvm/booke.c
> > > > > index ab62109..a5ee42c 100644
> > > > > --- a/arch/powerpc/kvm/booke.c
> > > > > +++ b/arch/powerpc/kvm/booke.c
> > > > > @@ -1804,8 +1804,6 @@ int
> > > > > kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu
> > > > *vcpu,
> > > > >  	kvm_guest_protect_msr(vcpu, MSR_DE, true);
> > > > >  	vcpu->guest_debug = dbg->control;
> > > > >  	vcpu->arch.shadow_dbg_reg.dbcr0 = 0;
> > > > > -	/* Set DBCR0_EDM in guest visible DBCR0 register. */
> > > > > -	vcpu->arch.dbg_reg.dbcr0 = DBCR0_EDM;
> > > > >
> > > > >  	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)
> > > > >  		vcpu->arch.shadow_dbg_reg.dbcr0 |= DBCR0_IDM | DBCR0_IC;
> > > >
> > > > This was intended to let the guest know that the host owns the debug
> > > > resources, by analogy to what a JTAG debugger would do.
> > > >
> > > > The Power ISA has this "Virtualized Implementation Note":
> > > >
> > > >         It is the responsibility of the hypervisor to ensure that
> > > >         DBCR0[EDM] is consistent with usage of DEP.
> > >
> > > Ok, That means that if MSRP_DEP is set then set DBCR0_EDM  and if MSRP_DEP is
> > clear then clear DBCR0_EDM, right?
> > > We need to implement above mentioned this.
> > 
> > We should probably clear EDM only when guest debug emulation is working and
> > enabled (i.e. not until at least patch 6/6).
> 
> But if EDM is set then guest debug emulation will not start/allowed.

I don't mean after the guest tries to write to the registers -- I mean
after the code has been added to KVM to allow it to work.

-Scott