KVM: arm64: Force SRE traps when SRE access is not enabled

We so far only write the ICH_HCR_EL2 config in two situations:

- when we need to emulate the GICv3 CPU interface due to HW bugs

- when we do direct injection, as the virtual CPU interface needs
  to be enabled

This is all good. But it also means that we don't do anything special
when we emulate a GICv2, or that there is no GIC at all.

What happens in this case when the guest uses the GICv3 system
registers? The *guest* gets a trap for a sysreg access (EC=0x18)
while we'd really like it to get an UNDEF.

Fixing this is a bit involved:

- we need to set all the required trap bits (TC, TALL0, TALL1, TDIR)

- for these traps to take effect, we need to (counter-intuitively)
  set ICC_SRE_EL1.SRE to 1 so that the above traps take priority.

Note that doesn't fully work when GICv2 emulation is enabled, as
we cannot set ICC_SRE_EL1.SRE to 1 (it breaks Group0 delivery as
IRQ).
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240827152517.3909653-3-maz@kernel.orgSigned-off-by: Marc Zyngier <maz@kernel.org>

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

@@ -268,8 +268,16 @@ void __vgic_v3_activate_traps(struct vgic_v3_cpu_if *cpu_if)
 	 * starting to mess with the rest of the GIC, and VMCR_EL2 in
 	 * particular.  This logic must be called before
 	 * __vgic_v3_restore_state().
+	 *
+	 * However, if the vgic is disabled (ICH_HCR_EL2.EN==0), no GIC is
+	 * provisioned at all. In order to prevent illegal accesses to the
+	 * system registers to trap to EL1 (duh), force ICC_SRE_EL1.SRE to 1
+	 * so that the trap bits can take effect. Yes, we *loves* the GIC.
 	 */
-	if (!cpu_if->vgic_sre) {
+	if (!(cpu_if->vgic_hcr & ICH_HCR_EN)) {
+		write_gicreg(ICC_SRE_EL1_SRE, ICC_SRE_EL1);
+		isb();
+	} else if (!cpu_if->vgic_sre) {
 		write_gicreg(0, ICC_SRE_EL1);
 		isb();
 		write_gicreg(cpu_if->vgic_vmcr, ICH_VMCR_EL2);
@@ -288,8 +296,9 @@ void __vgic_v3_activate_traps(struct vgic_v3_cpu_if *cpu_if)
 	}
 
 	/*
-	 * Prevent the guest from touching the GIC system registers if
-	 * SRE isn't enabled for GICv3 emulation.
+	 * 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);
@@ -297,10 +306,11 @@ void __vgic_v3_activate_traps(struct vgic_v3_cpu_if *cpu_if)
 	/*
 	 * If we need to trap system registers, we must write
 	 * ICH_HCR_EL2 anyway, even if no interrupts are being
-	 * injected,
+	 * injected. Note that this also applies if we don't expect
+	 * any system register access (no vgic at all).
 	 */
 	if (static_branch_unlikely(&vgic_v3_cpuif_trap) ||
-	    cpu_if->its_vpe.its_vm)
+	    cpu_if->its_vpe.its_vm || !cpu_if->vgic_sre)
 		write_gicreg(cpu_if->vgic_hcr, ICH_HCR_EL2);
 }
 
@@ -326,7 +336,7 @@ void __vgic_v3_deactivate_traps(struct vgic_v3_cpu_if *cpu_if)
 	 * no interrupts were being injected, and we disable it again here.
 	 */
 	if (static_branch_unlikely(&vgic_v3_cpuif_trap) ||
-	    cpu_if->its_vpe.its_vm)
+	    cpu_if->its_vpe.its_vm || !cpu_if->vgic_sre)
 		write_gicreg(0, ICH_HCR_EL2);
 }
 

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

@@ -298,8 +298,11 @@ void vcpu_set_ich_hcr(struct kvm_vcpu *vcpu)
 {
 	struct vgic_v3_cpu_if *vgic_v3 = &vcpu->arch.vgic_cpu.vgic_v3;
 
-	if (!kvm_has_gicv3(vcpu->kvm))
+	/* Hide GICv3 sysreg if necessary */
+	if (!kvm_has_gicv3(vcpu->kvm)) {
+		vgic_v3->vgic_hcr |= ICH_HCR_TALL0 | ICH_HCR_TALL1 | ICH_HCR_TC;
 		return;
+	}
 
 	if (group0_trap)
 		vgic_v3->vgic_hcr |= ICH_HCR_TALL0;