KVM: PPC: Book3S HV: Avoid returning to nested hypervisor on pending doorbells

[ Upstream commit 26686db699 ]

Commit 6398326b9b ("KVM: PPC: Book3S HV P9: Stop using vc->dpdes")
dropped the use of vcore->dpdes for msgsndp / SMT emulation. Prior to that
commit, the below code at L1 level (see [1] for terminology) was
responsible for setting vc->dpdes for the respective L2 vCPU:

if (!nested) {
	kvmppc_core_prepare_to_enter(vcpu);
	if (vcpu->arch.doorbell_request) {
		vc->dpdes = 1;
		smp_wmb();
		vcpu->arch.doorbell_request = 0;
	}

L1 then sent vc->dpdes to L0 via kvmhv_save_hv_regs(), and while
servicing H_ENTER_NESTED at L0, the below condition at L0 level made sure
to abort and go back to L1 if vcpu->arch.doorbell_request = 1 so that L1
sets vc->dpdes as per above if condition:

} else if (vcpu->arch.pending_exceptions ||
	   vcpu->arch.doorbell_request ||
	   xive_interrupt_pending(vcpu)) {
	vcpu->arch.ret = RESUME_HOST;
	goto out;
}

This worked fine since vcpu->arch.doorbell_request was used more like a
flag and vc->dpdes was used to pass around the doorbell state. But after
Commit 6398326b9b ("KVM: PPC: Book3S HV P9: Stop using vc->dpdes"),
vcpu->arch.doorbell_request is the only variable used to pass around
doorbell state.
With the plumbing for handling doorbells for nested guests updated to use
vcpu->arch.doorbell_request over vc->dpdes, the above "else if" stops
doorbells from working correctly as L0 aborts execution of L2 and
instead goes back to L1.

Remove vcpu->arch.doorbell_request from the above "else if" condition as
it is no longer needed for L0 to correctly handle the doorbell status
while running L2.

[1] Terminology
1. L0 : PowerNV linux running with HV privileges
2. L1 : Pseries KVM guest running on top of L0
2. L2 : Nested KVM guest running on top of L1

Fixes: 6398326b9b ("KVM: PPC: Book3S HV P9: Stop using vc->dpdes")
Signed-off-by: Gautam Menghani <gautam@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20241109063301.105289-4-gautam@linux.ibm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

arch/powerpc/kvm/book3s_hv.c

@@ -4687,7 +4687,6 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit,
 				lpcr |= LPCR_MER;
 		}
 	} else if (vcpu->arch.pending_exceptions ||
-		   vcpu->arch.doorbell_request ||
 		   xive_interrupt_pending(vcpu)) {
 		vcpu->arch.ret = RESUME_HOST;
 		goto out;