linux-kernel - Re: [PATCH 2/2] KVM: PPC: Book3S HV: H_SVM_INIT_START must call UV

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Message-Id: <20200323042114.GH26049@in.ibm.com>
Date:   Mon, 23 Mar 2020 09:51:14 +0530
From:   Bharata B Rao <bharata@...ux.ibm.com>
To:     Laurent Dufour <ldufour@...ux.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        kvm-ppc@...r.kernel.org, Paul Mackerras <paulus@...abs.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 2/2] KVM: PPC: Book3S HV: H_SVM_INIT_START must call
 UV_RETURN

On Fri, Mar 20, 2020 at 03:36:05PM +0100, Laurent Dufour wrote:
> Le 20/03/2020 à 12:24, Bharata B Rao a écrit :
> > On Fri, Mar 20, 2020 at 11:26:43AM +0100, Laurent Dufour wrote:
> > > When the call to UV_REGISTER_MEM_SLOT is failing, for instance because
> > > there is not enough free secured memory, the Hypervisor (HV) has to call
> > > UV_RETURN to report the error to the Ultravisor (UV). Then the UV will call
> > > H_SVM_INIT_ABORT to abort the securing phase and go back to the calling VM.
> > > 
> > > If the kvm->arch.secure_guest is not set, in the return path rfid is called
> > > but there is no valid context to get back to the SVM since the Hcall has
> > > been routed by the Ultravisor.
> > > 
> > > Move the setting of kvm->arch.secure_guest earlier in
> > > kvmppc_h_svm_init_start() so in the return path, UV_RETURN will be called
> > > instead of rfid.
> > > 
> > > Cc: Bharata B Rao <bharata@...ux.ibm.com>
> > > Cc: Paul Mackerras <paulus@...abs.org>
> > > Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> > > Cc: Michael Ellerman <mpe@...erman.id.au>
> > > Signed-off-by: Laurent Dufour <ldufour@...ux.ibm.com>
> > > ---
> > >   arch/powerpc/kvm/book3s_hv_uvmem.c | 3 ++-
> > >   1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
> > > index 79b1202b1c62..68dff151315c 100644
> > > --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
> > > +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
> > > @@ -209,6 +209,8 @@ unsigned long kvmppc_h_svm_init_start(struct kvm *kvm)
> > >   	int ret = H_SUCCESS;
> > >   	int srcu_idx;
> > > +	kvm->arch.secure_guest = KVMPPC_SECURE_INIT_START;
> > > +
> > >   	if (!kvmppc_uvmem_bitmap)
> > >   		return H_UNSUPPORTED;
> > > @@ -233,7 +235,6 @@ unsigned long kvmppc_h_svm_init_start(struct kvm *kvm)
> > >   			goto out;
> > >   		}
> > >   	}
> > > -	kvm->arch.secure_guest |= KVMPPC_SECURE_INIT_START;
> > 
> > There is an assumption that memory slots would have been registered with UV
> > if KVMPPC_SECURE_INIT_START has been done. KVM_PPC_SVM_OFF ioctl will skip
> > unregistration and other steps during reboot if KVMPPC_SECURE_INIT_START
> > hasn't been done.
> > 
> > Have you checked if that path isn't affected by this change?
> 
> I checked that and didn't find any issue there.
> 
> My only concern was that block:
> 	kvm_for_each_vcpu(i, vcpu, kvm) {
> 		spin_lock(&vcpu->arch.vpa_update_lock);
> 		unpin_vpa_reset(kvm, &vcpu->arch.dtl);
> 		unpin_vpa_reset(kvm, &vcpu->arch.slb_shadow);
> 		unpin_vpa_reset(kvm, &vcpu->arch.vpa);
> 		spin_unlock(&vcpu->arch.vpa_update_lock);
> 	}
> 
> But that seems to be safe.

Yes, looks like.

> 
> However I'm not a familiar with the KVM's code, do you think an additional
> KVMPPC_SECURE_INIT_* value needed here?

May be not as long as UV can handle the unexpected uv_unregister_mem_slot()
calls, we are good.

Regards,
Bharata.