KVM: arm64: nv: Reinject traps that take effect in Host EL0

Wire up the other end of traps that affect host EL0 by actually
injecting them into the guest hypervisor. Skip over FGT entirely, as a
cursory glance suggests no FGT is effective in host EL0.

Note that kvm_inject_nested() is already equipped for handling
exceptions while the VM is already in a host context.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20241025182354.3364124-9-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

Author: oupton

arch/arm64/include/asm/kvm_emulate.h

@@ -225,6 +225,11 @@ static inline bool is_hyp_ctxt(const struct kvm_vcpu *vcpu)
 	return vcpu_has_nv(vcpu) && __is_hyp_ctxt(&vcpu->arch.ctxt);
 }
 
+static inline bool vcpu_is_host_el0(const struct kvm_vcpu *vcpu)
+{
+	return is_hyp_ctxt(vcpu) && !vcpu_is_el2(vcpu);
+}
+
 /*
  * The layout of SPSR for an AArch32 state is different when observed from an
  * AArch64 SPSR_ELx or an AArch32 SPSR_*. This function generates the AArch32

arch/arm64/kvm/emulate-nested.c

@@ -20,6 +20,9 @@ enum trap_behaviour {
 	BEHAVE_FORWARD_READ	= BIT(0),
 	BEHAVE_FORWARD_WRITE	= BIT(1),
 	BEHAVE_FORWARD_RW	= BEHAVE_FORWARD_READ | BEHAVE_FORWARD_WRITE,
+
+	/* Traps that take effect in Host EL0, this is rare! */
+	BEHAVE_FORWARD_IN_HOST_EL0	= BIT(2),
 };
 
 struct trap_bits {
@@ -2128,11 +2131,19 @@ static u64 kvm_get_sysreg_res0(struct kvm *kvm, enum vcpu_sysreg sr)
 	return masks->mask[sr - __VNCR_START__].res0;
 }
 
-static bool check_fgt_bit(struct kvm *kvm, bool is_read,
+static bool check_fgt_bit(struct kvm_vcpu *vcpu, bool is_read,
 			  u64 val, const union trap_config tc)
 {
+	struct kvm *kvm = vcpu->kvm;
 	enum vcpu_sysreg sr;
 
+	/*
+	 * KVM doesn't know about any FGTs that apply to the host, and hopefully
+	 * that'll remain the case.
+	 */
+	if (is_hyp_ctxt(vcpu))
+		return false;
+
 	if (tc.pol)
 		return (val & BIT(tc.bit));
 
@@ -2209,7 +2220,15 @@ bool triage_sysreg_trap(struct kvm_vcpu *vcpu, int *sr_index)
 	 * If we're not nesting, immediately return to the caller, with the
 	 * sysreg index, should we have it.
 	 */
-	if (!vcpu_has_nv(vcpu) || is_hyp_ctxt(vcpu))
+	if (!vcpu_has_nv(vcpu))
+		goto local;
+
+	/*
+	 * There are a few traps that take effect InHost, but are constrained
+	 * to EL0. Don't bother with computing the trap behaviour if the vCPU
+	 * isn't in EL0.
+	 */
+	if (is_hyp_ctxt(vcpu) && !vcpu_is_host_el0(vcpu))
 		goto local;
 
 	switch ((enum fgt_group_id)tc.fgt) {
@@ -2255,12 +2274,14 @@ bool triage_sysreg_trap(struct kvm_vcpu *vcpu, int *sr_index)
 		goto local;
 	}
 
-	if (tc.fgt != __NO_FGT_GROUP__ && check_fgt_bit(vcpu->kvm, is_read,
-							val, tc))
+	if (tc.fgt != __NO_FGT_GROUP__ && check_fgt_bit(vcpu, is_read, val, tc))
 		goto inject;
 
 	b = compute_trap_behaviour(vcpu, tc);
 
+	if (!(b & BEHAVE_FORWARD_IN_HOST_EL0) && vcpu_is_host_el0(vcpu))
+		goto local;
+
 	if (((b & BEHAVE_FORWARD_READ) && is_read) ||
 	    ((b & BEHAVE_FORWARD_WRITE) && !is_read))
 		goto inject;