[PATCH v2 18/28] KVM: arm64: nv: Emulate CMOs via writable-clean translations as NOPs

[Oliver Upton <oupton@kernel.org>]

Virtualizing FEAT_HAFDBS for an L1 hypervisor has a pretty large
architectural deficiency by way of cache maintenance instructions.
CMOs (e.g. DC IVAC) require write permission but do not set the dirty
state on the descriptor.

There's no way to shadow a writable-clean translation in such a way that
these CMOs pass the permission check and subsequent writes generate a
permission fault. Worse yet, data aborts taken on CMOs do not have
sufficient syndrome information to perform instruction emulation and
trapping all CMOs is unpalatable.

So what do we do? Absolutely nothing. Treat CMOs to writable-clean
translations as NOPs in anticipation of KVM only exposing FEAT_HAFDBS
when the VM is fully coherent (FEAT_S2FWB, CTR_EL0.{DIC,IDC}={1,1}, no
noncoherent DMA).

Signed-off-by: Oliver Upton <oupton@kernel.org>
---
 arch/arm64/kvm/mmu.c    |  4 ++++
 arch/arm64/kvm/nested.c | 20 ++++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 7b29be33a1ea..a21b674a1981 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -2548,6 +2548,10 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu)
 		}
 
 		ret = kvm_s2_handle_perm_fault(vcpu, &nested_trans);
+		if (ret == -EAGAIN) {
+			ret = 1;
+			goto out_unlock;
+		}
 		if (ret) {
 			esr = nested_trans.esr;
 			kvm_inject_s2_fault(vcpu, esr);
diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index 7dda95d37925..46fcc75f4c20 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -1150,6 +1150,26 @@ int kvm_s2_handle_perm_fault(struct kvm_vcpu *vcpu, struct kvm_s2_trans *trans)
 	} else {
 		bool write_fault = kvm_is_write_fault(vcpu);
 
+		/*
+		 * Plug your nose for a moment.
+		 *
+		 * Writable-clean translations are shadowed using a read-only
+		 * mapping so KVM can set the dirty state on the L1 descriptor
+		 * before installing a writable mapping.
+		 *
+		 * Certain cache maintenance instructions require a write
+		 * permission check but leave the translation in a writable-clean
+		 * state, meaning KVM is on the hook to emulate CMOs that use a
+		 * writable-clean translation.
+		 *
+		 * "Emulate" the CMO as a NOP, as KVM only exposes FEAT_HAFDBS
+		 * when FEAT_S2FWB is present and the I-cache is coherent.
+		 */
+		if (kvm_vcpu_dabt_is_cm(vcpu) && trans->writable && !trans->dirty) {
+			kvm_incr_pc(vcpu);
+			return -EAGAIN;
+		}
+
 		forward_fault = ((write_fault && !trans->writable) ||
 				 (!write_fault && !trans->readable));
 	}
-- 
2.47.3