KVM: arm64: Fail protected mode init if no vgic hardware is present

Protected mode assumes that at minimum vgic-v3 is present, however KVM
fails to actually enforce this at the time of initialization. As such,
when running protected mode in a half-baked state on GICv2 hardware we
see the hyp go belly up at vcpu_load() when it tries to restore the
vgic-v3 cpuif:

  $ ./arch_timer_edge_cases
  [  130.599140] kvm [4518]: nVHE hyp panic at: [<ffff800081102b58>] __kvm_nvhe___vgic_v3_restore_vmcr_aprs+0x8/0x84!
  [  130.603685] kvm [4518]: Cannot dump pKVM nVHE stacktrace: !CONFIG_PROTECTED_NVHE_STACKTRACE
  [  130.611962] kvm [4518]: Hyp Offset: 0xfffeca95ed000000
  [  130.617053] Kernel panic - not syncing: HYP panic:
  [  130.617053] PS:800003c9 PC:0000b56a94102b58 ESR:0000000002000000
  [  130.617053] FAR:ffff00007b98d4d0 HPFAR:00000000007b98d0 PAR:0000000000000000
  [  130.617053] VCPU:0000000000000000
  [  130.638013] CPU: 0 UID: 0 PID: 4518 Comm: arch_timer_edge Tainted: G         C         6.13.0-rc3-00009-gf7d03fcbf1f4 #1
  [  130.648790] Tainted: [C]=CRAP
  [  130.651721] Hardware name: Libre Computer AML-S905X-CC (DT)
  [  130.657242] Call trace:
  [  130.659656]  show_stack+0x18/0x24 (C)
  [  130.663279]  dump_stack_lvl+0x38/0x90
  [  130.666900]  dump_stack+0x18/0x24
  [  130.670178]  panic+0x388/0x3e8
  [  130.673196]  nvhe_hyp_panic_handler+0x104/0x208
  [  130.677681]  kvm_arch_vcpu_load+0x290/0x548
  [  130.681821]  vcpu_load+0x50/0x80
  [  130.685013]  kvm_arch_vcpu_ioctl_run+0x30/0x868
  [  130.689498]  kvm_vcpu_ioctl+0x2e0/0x974
  [  130.693293]  __arm64_sys_ioctl+0xb4/0xec
  [  130.697174]  invoke_syscall+0x48/0x110
  [  130.700883]  el0_svc_common.constprop.0+0x40/0xe0
  [  130.705540]  do_el0_svc+0x1c/0x28
  [  130.708818]  el0_svc+0x30/0xd0
  [  130.711837]  el0t_64_sync_handler+0x10c/0x138
  [  130.716149]  el0t_64_sync+0x198/0x19c
  [  130.719774] SMP: stopping secondary CPUs
  [  130.723660] Kernel Offset: disabled
  [  130.727103] CPU features: 0x000,00000800,02800000,0200421b
  [  130.732537] Memory Limit: none
  [  130.735561] ---[ end Kernel panic - not syncing: HYP panic:
  [  130.735561] PS:800003c9 PC:0000b56a94102b58 ESR:0000000002000000
  [  130.735561] FAR:ffff00007b98d4d0 HPFAR:00000000007b98d0 PAR:0000000000000000
  [  130.735561] VCPU:0000000000000000 ]---

Fix it by failing KVM initialization if the system doesn't implement
vgic-v3, as protected mode will never do anything useful on such
hardware.

Reported-by: Mark Brown <broonie@kernel.org>
Closes: https://lore.kernel.org/kvmarm/5ca7588c-7bf2-4352-8661-e4a56a9cd9aa@sirena.org.uk/
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250203231543.233511-1-oliver.upton@linux.dev
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/arm.c

@@ -2290,6 +2290,19 @@ static int __init init_subsystems(void)
 		break;
 	case -ENODEV:
 	case -ENXIO:
+		/*
+		 * No VGIC? No pKVM for you.
+		 *
+		 * Protected mode assumes that VGICv3 is present, so no point
+		 * in trying to hobble along if vgic initialization fails.
+		 */
+		if (is_protected_kvm_enabled())
+			goto out;
+
+		/*
+		 * Otherwise, userspace could choose to implement a GIC for its
+		 * guest on non-cooperative hardware.
+		 */
 		vgic_present = false;
 		err = 0;
 		break;