linux-kernel - [RFC PATCH 2/2] arch/powerpc/kvm: Fix doorbells for nested KVM guests on PowerNV

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Date: Thu, 27 Jun 2024 23:33:36 +0530
From: Gautam Menghani <gautam@...ux.ibm.com>
To: mpe@...erman.id.au, npiggin@...il.com, christophe.leroy@...roup.eu,
        naveen.n.rao@...ux.ibm.com
Cc: Gautam Menghani <gautam@...ux.ibm.com>, linuxppc-dev@...ts.ozlabs.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [RFC PATCH 2/2] arch/powerpc/kvm: Fix doorbells for nested KVM guests on PowerNV

commit 6398326b9ba1("KVM: PPC: Book3S HV P9: Stop using vc->dpdes")
introduced an optimization to use only vcpu->doorbell_request for SMT
emulation for Power9 and above guests, but the code for nested guests 
still relies on the old way of handling doorbells, due to which an L2
guest cannot be booted with XICS with SMT>1. The command to repro
this issue is:

qemu-system-ppc64 \
	-drive file=rhel.qcow2,format=qcow2 \
	-m 20G \
	-smp 8,cores=1,threads=8 \
	-cpu  host \
	-nographic \
	-machine pseries,ic-mode=xics -accel kvm

Fix the plumbing to utilize vcpu->doorbell_request instead of vcore->dpdes 
on P9 and above.

Fixes: 6398326b9ba1 ("KVM: PPC: Book3S HV P9: Stop using vc->dpdes")
Signed-off-by: Gautam Menghani <gautam@...ux.ibm.com>
---
 arch/powerpc/kvm/book3s_hv.c        |  9 ++++++++-
 arch/powerpc/kvm/book3s_hv_nested.c | 20 ++++++++++++++++----
 2 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index cea28ac05923..0586fa636707 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -4178,6 +4178,9 @@ static int kvmhv_vcpu_entry_p9_nested(struct kvm_vcpu *vcpu, u64 time_limit, uns
 	}
 	hvregs.hdec_expiry = time_limit;
 
+	// clear doorbell bit as hvregs already has the info
+	vcpu->arch.doorbell_request = 0;
+
 	/*
 	 * When setting DEC, we must always deal with irq_work_raise
 	 * via NMI vs setting DEC. The problem occurs right as we
@@ -4694,6 +4697,7 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit,
 	struct kvm_nested_guest *nested = vcpu->arch.nested;
 	unsigned long flags;
 	u64 tb;
+	bool doorbell_pending;
 
 	trace_kvmppc_run_vcpu_enter(vcpu);
 
@@ -4752,6 +4756,9 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit,
 	 */
 	smp_mb();
 
+	doorbell_pending = !cpu_has_feature(CPU_FTR_ARCH_300) &&
+				vcpu->arch.doorbell_request;
+
 	if (!nested) {
 		kvmppc_core_prepare_to_enter(vcpu);
 		if (test_bit(BOOK3S_IRQPRIO_EXTERNAL,
@@ -4769,7 +4776,7 @@ 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 ||
+		   doorbell_pending ||
 		   xive_interrupt_pending(vcpu)) {
 		vcpu->arch.ret = RESUME_HOST;
 		goto out;
diff --git a/arch/powerpc/kvm/book3s_hv_nested.c b/arch/powerpc/kvm/book3s_hv_nested.c
index 05f5220960c6..b34eefa6b268 100644
--- a/arch/powerpc/kvm/book3s_hv_nested.c
+++ b/arch/powerpc/kvm/book3s_hv_nested.c
@@ -32,7 +32,10 @@ void kvmhv_save_hv_regs(struct kvm_vcpu *vcpu, struct hv_guest_state *hr)
 	struct kvmppc_vcore *vc = vcpu->arch.vcore;
 
 	hr->pcr = vc->pcr | PCR_MASK;
-	hr->dpdes = vc->dpdes;
+	if (cpu_has_feature(CPU_FTR_ARCH_300))
+		hr->dpdes = vcpu->arch.doorbell_request;
+	else
+		hr->dpdes = vc->dpdes;
 	hr->hfscr = vcpu->arch.hfscr;
 	hr->tb_offset = vc->tb_offset;
 	hr->dawr0 = vcpu->arch.dawr0;
@@ -105,7 +108,10 @@ static void save_hv_return_state(struct kvm_vcpu *vcpu,
 {
 	struct kvmppc_vcore *vc = vcpu->arch.vcore;
 
-	hr->dpdes = vc->dpdes;
+	if (cpu_has_feature(CPU_FTR_ARCH_300))
+		hr->dpdes = vcpu->arch.doorbell_request;
+	else
+		hr->dpdes = vc->dpdes;
 	hr->purr = vcpu->arch.purr;
 	hr->spurr = vcpu->arch.spurr;
 	hr->ic = vcpu->arch.ic;
@@ -143,7 +149,10 @@ static void restore_hv_regs(struct kvm_vcpu *vcpu, const struct hv_guest_state *
 	struct kvmppc_vcore *vc = vcpu->arch.vcore;
 
 	vc->pcr = hr->pcr | PCR_MASK;
-	vc->dpdes = hr->dpdes;
+	if (cpu_has_feature(CPU_FTR_ARCH_300))
+		vcpu->arch.doorbell_request = hr->dpdes;
+	else
+		vc->dpdes = hr->dpdes;
 	vcpu->arch.hfscr = hr->hfscr;
 	vcpu->arch.dawr0 = hr->dawr0;
 	vcpu->arch.dawrx0 = hr->dawrx0;
@@ -170,7 +179,10 @@ void kvmhv_restore_hv_return_state(struct kvm_vcpu *vcpu,
 {
 	struct kvmppc_vcore *vc = vcpu->arch.vcore;
 
-	vc->dpdes = hr->dpdes;
+	if (cpu_has_feature(CPU_FTR_ARCH_300) && !vcpu->arch.doorbell_request)
+		vcpu->arch.doorbell_request = hr->dpdes;
+	else
+		vc->dpdes = hr->dpdes;
 	vcpu->arch.hfscr = hr->hfscr;
 	vcpu->arch.purr = hr->purr;
 	vcpu->arch.spurr = hr->spurr;
-- 
2.45.1