Patch "KVM: PPC: Book3S HV: Stop using vc->dpdes for nested KVM guests" has been added to the 6.1-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    KVM: PPC: Book3S HV: Stop using vc->dpdes for nested KVM guests

to the 6.1-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     kvm-ppc-book3s-hv-stop-using-vc-dpdes-for-nested-kvm.patch
and it can be found in the queue-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit e65b5968f57c4e7534aa334ed8cca2b7de573db1
Author: Gautam Menghani <gautam@xxxxxxxxxxxxx>
Date:   Sat Nov 9 12:02:56 2024 +0530

    KVM: PPC: Book3S HV: Stop using vc->dpdes for nested KVM guests
    
    [ Upstream commit 0d3c6b28896f9889c8864dab469e0343a0ad1c0c ]
    
    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 (see [1]) cannot be booted with XICS with SMT>1. The command to
    repro this issue is:
    
    // To be run in L1
    
    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
    for nested KVM guests on P9 and above.
    
    [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: 6398326b9ba1 ("KVM: PPC: Book3S HV P9: Stop using vc->dpdes")
    Signed-off-by: Gautam Menghani <gautam@xxxxxxxxxxxxx>
    Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
    Link: https://patch.msgid.link/20241109063301.105289-3-gautam@xxxxxxxxxxxxx
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 6ba68dd6190bd..479218389e7cd 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -4062,6 +4062,15 @@ static int kvmhv_vcpu_entry_p9_nested(struct kvm_vcpu *vcpu, u64 time_limit, uns
 	}
 	hvregs.hdec_expiry = time_limit;
 
+	/*
+	 * hvregs has the doorbell status, so zero it here which
+	 * enables us to receive doorbells when H_ENTER_NESTED is
+	 * in progress for this vCPU
+	 */
+
+	if (vcpu->arch.doorbell_request)
+		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
diff --git a/arch/powerpc/kvm/book3s_hv_nested.c b/arch/powerpc/kvm/book3s_hv_nested.c
index 5a64a1341e6f1..bc71a90c7cc76 100644
--- a/arch/powerpc/kvm/book3s_hv_nested.c
+++ b/arch/powerpc/kvm/book3s_hv_nested.c
@@ -32,7 +32,7 @@ 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;
+	hr->dpdes = vcpu->arch.doorbell_request;
 	hr->hfscr = vcpu->arch.hfscr;
 	hr->tb_offset = vc->tb_offset;
 	hr->dawr0 = vcpu->arch.dawr0;
@@ -105,7 +105,7 @@ static void save_hv_return_state(struct kvm_vcpu *vcpu,
 {
 	struct kvmppc_vcore *vc = vcpu->arch.vcore;
 
-	hr->dpdes = vc->dpdes;
+	hr->dpdes = vcpu->arch.doorbell_request;
 	hr->purr = vcpu->arch.purr;
 	hr->spurr = vcpu->arch.spurr;
 	hr->ic = vcpu->arch.ic;
@@ -143,7 +143,7 @@ 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;
+	vcpu->arch.doorbell_request = hr->dpdes;
 	vcpu->arch.hfscr = hr->hfscr;
 	vcpu->arch.dawr0 = hr->dawr0;
 	vcpu->arch.dawrx0 = hr->dawrx0;
@@ -170,7 +170,13 @@ void kvmhv_restore_hv_return_state(struct kvm_vcpu *vcpu,
 {
 	struct kvmppc_vcore *vc = vcpu->arch.vcore;
 
-	vc->dpdes = hr->dpdes;
+	/*
+	 * This L2 vCPU might have received a doorbell while H_ENTER_NESTED was being handled.
+	 * Make sure we preserve the doorbell if it was either:
+	 *   a) Sent after H_ENTER_NESTED was called on this vCPU (arch.doorbell_request would be 1)
+	 *   b) Doorbell was not handled and L2 exited for some other reason (hr->dpdes would be 1)
+	 */
+	vcpu->arch.doorbell_request = vcpu->arch.doorbell_request | hr->dpdes;
 	vcpu->arch.hfscr = hr->hfscr;
 	vcpu->arch.purr = hr->purr;
 	vcpu->arch.spurr = hr->spurr;