On Fri, Nov 29, 2013 at 03:54:58AM +0100, Alexander Graf wrote: > We call a C helper to save all svcpu fields into our vcpu. The C > ABI states that r12 is considered volatile. However, we keep our > exit handler id in r12 currently. > > So we need to save it away into a non-volatile register instead > that definitely does get preserved across the C call. > > This bug usually didn't hit anyone yet since gcc is smart enough > to generate code that doesn't even need r12 which means it stayed > identical throughout the call by sheer luck. But we can't rely on > that. > > Signed-off-by: Alexander Graf <agraf@xxxxxxx> > --- > arch/powerpc/kvm/book3s_interrupts.S | 15 +++++++++++---- > 1 file changed, 11 insertions(+), 4 deletions(-) > > diff --git a/arch/powerpc/kvm/book3s_interrupts.S b/arch/powerpc/kvm/book3s_interrupts.S > index f4dd041..2f7d571 100644 > --- a/arch/powerpc/kvm/book3s_interrupts.S > +++ b/arch/powerpc/kvm/book3s_interrupts.S > @@ -132,9 +132,18 @@ kvm_start_lightweight: > * > */ > > + PPC_LL r3, GPR4(r1) /* vcpu pointer */ > + > + /* > + * kvmppc_copy_from_svcpu can clobber volatile registers, save > + * r14 to get a spare scratch register for the exit handler id. > + */ > + PPC_STL r14, VCPU_GPR(R14)(r3) > + mr r14, r12 In the case where kvmppc_handle_exit_pr returns RESUME_GUEST and we do the lightweight guest re-entry, I don't see anywhere that we re-load r14 with the guest value. So I think this results in guest-visible corruption of r14. Why not just use the vcpu->arch.trap field? Store r12 to VCPU_TRAP(r3) here and load r5 from VCPU_TRAP(r4) just before calling kvmppc_handle_exit_pr(). Regards, Paul. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
