KVM: arm64: gic-v5: Support GICv3 compat

Add support for GICv3 compat mode (FEAT_GCIE_LEGACY) which allows a
GICv5 host to run GICv3-based VMs. This change enables the
VHE/nVHE/hVHE/protected modes, but does not support nested
virtualization.

A lazy-disable approach is taken for compat mode; it is enabled on the
vgic_v3_load path but not disabled on the vgic_v3_put path. A
non-GICv3 VM, i.e., one based on GICv5, is responsible for disabling
compat mode on the corresponding vgic_v5_load path. Currently, GICv5
is not supported, and hence compat mode is not disabled again once it
is enabled, and this function is intentionally omitted from the code.

Co-authored-by: Timothy Hayes <timothy.hayes@arm.com>
Signed-off-by: Timothy Hayes <timothy.hayes@arm.com>
Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
Link: https://lore.kernel.org/r/20250627100847.1022515-5-sascha.bischoff@arm.com
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

Author: 2 people

arch/arm64/kvm/hyp/vgic-v3-sr.c

@@ -296,12 +296,19 @@ void __vgic_v3_activate_traps(struct vgic_v3_cpu_if *cpu_if)
 	}
 
 	/*
-	 * Prevent the guest from touching the ICC_SRE_EL1 system
-	 * register. Note that this may not have any effect, as
-	 * ICC_SRE_EL2.Enable being RAO/WI is a valid implementation.
+	 * GICv5 BET0 FEAT_GCIE_LEGACY doesn't include ICC_SRE_EL2. This is due
+	 * to be relaxed in a future spec release, at which point this in
+	 * condition can be dropped.
 	 */
-	write_gicreg(read_gicreg(ICC_SRE_EL2) & ~ICC_SRE_EL2_ENABLE,
-		     ICC_SRE_EL2);
+	if (!cpus_have_final_cap(ARM64_HAS_GICV5_CPUIF)) {
+		/*
+		 * Prevent the guest from touching the ICC_SRE_EL1 system
+		 * register. Note that this may not have any effect, as
+		 * ICC_SRE_EL2.Enable being RAO/WI is a valid implementation.
+		 */
+		write_gicreg(read_gicreg(ICC_SRE_EL2) & ~ICC_SRE_EL2_ENABLE,
+			     ICC_SRE_EL2);
+	}
 
 	/*
 	 * If we need to trap system registers, we must write
@@ -322,8 +329,14 @@ void __vgic_v3_deactivate_traps(struct vgic_v3_cpu_if *cpu_if)
 		cpu_if->vgic_vmcr = read_gicreg(ICH_VMCR_EL2);
 	}
 
-	val = read_gicreg(ICC_SRE_EL2);
-	write_gicreg(val | ICC_SRE_EL2_ENABLE, ICC_SRE_EL2);
+	/*
+	 * Can be dropped in the future when GICv5 spec is relaxed. See comment
+	 * above.
+	 */
+	if (!cpus_have_final_cap(ARM64_HAS_GICV5_CPUIF)) {
+		val = read_gicreg(ICC_SRE_EL2);
+		write_gicreg(val | ICC_SRE_EL2_ENABLE, ICC_SRE_EL2);
+	}
 
 	if (!cpu_if->vgic_sre) {
 		/* Make sure ENABLE is set at EL2 before setting SRE at EL1 */
@@ -423,9 +436,19 @@ void __vgic_v3_init_lrs(void)
  */
 u64 __vgic_v3_get_gic_config(void)
 {
-	u64 val, sre = read_gicreg(ICC_SRE_EL1);
+	u64 val, sre;
 	unsigned long flags = 0;
 
+	/*
+	 * In compat mode, we cannot access ICC_SRE_EL1 at any EL
+	 * other than EL1 itself; just return the
+	 * ICH_VTR_EL2. ICC_IDR0_EL1 is only implemented on a GICv5
+	 * system, so we first check if we have GICv5 support.
+	 */
+	if (cpus_have_final_cap(ARM64_HAS_GICV5_CPUIF))
+		return read_gicreg(ICH_VTR_EL2);
+
+	sre = read_gicreg(ICC_SRE_EL1);
 	/*
 	 * To check whether we have a MMIO-based (GICv2 compatible)
 	 * CPU interface, we need to disable the system register
@@ -471,6 +494,16 @@ u64 __vgic_v3_get_gic_config(void)
 	return val;
 }
 
+static void __vgic_v3_compat_mode_enable(void)
+{
+	if (!cpus_have_final_cap(ARM64_HAS_GICV5_CPUIF))
+		return;
+
+	sysreg_clear_set_s(SYS_ICH_VCTLR_EL2, 0, ICH_VCTLR_EL2_V3);
+	/* Wait for V3 to become enabled */
+	isb();
+}
+
 static u64 __vgic_v3_read_vmcr(void)
 {
 	return read_gicreg(ICH_VMCR_EL2);
@@ -490,6 +523,8 @@ void __vgic_v3_save_vmcr_aprs(struct vgic_v3_cpu_if *cpu_if)
 
 void __vgic_v3_restore_vmcr_aprs(struct vgic_v3_cpu_if *cpu_if)
 {
+	__vgic_v3_compat_mode_enable();
+
 	/*
 	 * If dealing with a GICv2 emulation on GICv3, VMCR_EL2.VFIQen
 	 * is dependent on ICC_SRE_EL1.SRE, and we have to perform the

arch/arm64/kvm/sys_regs.c

@@ -1811,7 +1811,7 @@ static u64 sanitise_id_aa64pfr0_el1(const struct kvm_vcpu *vcpu, u64 val)
 		val |= SYS_FIELD_PREP_ENUM(ID_AA64PFR0_EL1, CSV3, IMP);
 	}
 
-	if (kvm_vgic_global_state.type == VGIC_V3) {
+	if (vgic_is_v3(vcpu->kvm)) {
 		val &= ~ID_AA64PFR0_EL1_GIC_MASK;
 		val |= SYS_FIELD_PREP_ENUM(ID_AA64PFR0_EL1, GIC, IMP);
 	}
@@ -1953,6 +1953,14 @@ static int set_id_aa64pfr0_el1(struct kvm_vcpu *vcpu,
 	    (vcpu_has_nv(vcpu) && !FIELD_GET(ID_AA64PFR0_EL1_EL2, user_val)))
 		return -EINVAL;
 
+	/*
+	 * If we are running on a GICv5 host and support FEAT_GCIE_LEGACY, then
+	 * we support GICv3. Fail attempts to do anything but set that to IMP.
+	 */
+	if (vgic_is_v3_compat(vcpu->kvm) &&
+	    FIELD_GET(ID_AA64PFR0_EL1_GIC_MASK, user_val) != ID_AA64PFR0_EL1_GIC_IMP)
+		return -EINVAL;
+
 	return set_id_reg(vcpu, rd, user_val);
 }
 

arch/arm64/kvm/vgic/vgic-init.c

@@ -674,10 +674,12 @@ void kvm_vgic_init_cpu_hardware(void)
 	 * We want to make sure the list registers start out clear so that we
 	 * only have the program the used registers.
 	 */
-	if (kvm_vgic_global_state.type == VGIC_V2)
+	if (kvm_vgic_global_state.type == VGIC_V2) {
 		vgic_v2_init_lrs();
-	else
+	} else if (kvm_vgic_global_state.type == VGIC_V3 ||
+		   kvm_vgic_global_state.has_gcie_v3_compat) {
 		kvm_call_hyp(__vgic_v3_init_lrs);
+	}
 }
 
 /**

arch/arm64/kvm/vgic/vgic.h

@@ -389,6 +389,17 @@ void vgic_v3_put_nested(struct kvm_vcpu *vcpu);
 void vgic_v3_handle_nested_maint_irq(struct kvm_vcpu *vcpu);
 void vgic_v3_nested_update_mi(struct kvm_vcpu *vcpu);
 
+static inline bool vgic_is_v3_compat(struct kvm *kvm)
+{
+	return cpus_have_final_cap(ARM64_HAS_GICV5_CPUIF) &&
+		kvm_vgic_global_state.has_gcie_v3_compat;
+}
+
+static inline bool vgic_is_v3(struct kvm *kvm)
+{
+	return kvm_vgic_global_state.type == VGIC_V3 || vgic_is_v3_compat(kvm);
+}
+
 int vgic_its_debug_init(struct kvm_device *dev);
 void vgic_its_debug_destroy(struct kvm_device *dev);
 

include/kvm/arm_vgic.h

@@ -38,6 +38,7 @@
 enum vgic_type {
 	VGIC_V2,		/* Good ol' GICv2 */
 	VGIC_V3,		/* New fancy GICv3 */
+	VGIC_V5,		/* Newer, fancier GICv5 */
 };
 
 /* same for all guests, as depending only on the _host's_ GIC model */
@@ -77,9 +78,12 @@ struct vgic_global {
 	/* Pseudo GICv3 from outer space */
 	bool			no_hw_deactivation;
 
-	/* GIC system register CPU interface */
+	/* GICv3 system register CPU interface */
 	struct static_key_false gicv3_cpuif;
 
+	/* GICv3 compat mode on a GICv5 host */
+	bool			has_gcie_v3_compat;
+
 	u32			ich_vtr_el2;
 };