On 20/06/2019 03:46, Suraj Jitindar Singh wrote: > If we enter an L1 guest with a pending decrementer exception then this > is cleared on guest exit if the guest has writtien a positive value into > the decrementer (indicating that it handled the decrementer exception) > since there is no other way to detect that the guest has handled the > pending exception and that it should be dequeued. In the event that the > L1 guest tries to run a nested (L2) guest immediately after this and the > L2 guest decrementer is negative (which is loaded by L1 before making > the H_ENTER_NESTED hcall), then the pending decrementer exception > isn't cleared and the L2 entry is blocked since L1 has a pending > exception, even though L1 may have already handled the exception and > written a positive value for it's decrementer. This results in a loop of > L1 trying to enter the L2 guest and L0 blocking the entry since L1 has > an interrupt pending with the outcome being that L2 never gets to run > and hangs. > > Fix this by clearing any pending decrementer exceptions when L1 makes > the H_ENTER_NESTED hcall since it won't do this if it's decrementer has > gone negative, and anyway it's decrementer has been communicated to L0 > in the hdec_expires field and L0 will return control to L1 when this > goes negative by delivering an H_DECREMENTER exception. > > Fixes: 95a6432ce903 "KVM: PPC: Book3S HV: Streamlined guest entry/exit path > on P9 for radix guests" > > Signed-off-by: Suraj Jitindar Singh <sjitindarsi...@gmail.com> > --- > arch/powerpc/kvm/book3s_hv.c | 11 +++++++++-- > 1 file changed, 9 insertions(+), 2 deletions(-) > > diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c > index 719fd2529eec..4a5eb29b952f 100644 > --- a/arch/powerpc/kvm/book3s_hv.c > +++ b/arch/powerpc/kvm/book3s_hv.c > @@ -4128,8 +4128,15 @@ int kvmhv_run_single_vcpu(struct kvm_run *kvm_run, > > preempt_enable(); > > - /* cancel pending decrementer exception if DEC is now positive */ > - if (get_tb() < vcpu->arch.dec_expires && kvmppc_core_pending_dec(vcpu)) > + /* > + * cancel pending decrementer exception if DEC is now positive, or if > + * entering a nested guest in which case the decrementer is now owned > + * by L2 and the L1 decrementer is provided in hdec_expires > + */ > + if (kvmppc_core_pending_dec(vcpu) && > + ((get_tb() < vcpu->arch.dec_expires) || > + (trap == BOOK3S_INTERRUPT_SYSCALL && > + kvmppc_get_gpr(vcpu, 3) == H_ENTER_NESTED))) > kvmppc_core_dequeue_dec(vcpu); > > trace_kvm_guest_exit(vcpu); >
Patches 2 and 3: tested I can boot and run an L2 nested guest with qemu v4.0.0 and caps-large-decr=on in the case we have had a hang previously. Tested-by: Laurent Vivier <lviv...@redhat.com>
