KVM: arm64: Finalize ID registers only once per VM

Owing to the ID registers being global to the VM, there is no point
in computing them more than once.  However, recent changes making
use of kvm_set_vm_id_reg() outlined that we repeatedly hammer
the ID registers when we shouldn't.

Gate the ID reg update on the VM having never run.

Fixes: 50e7cce81b9b2 ("KVM: arm64: Limit clearing of ID_{AA64PFR0,PFR1}_EL1.GIC to userspace irqchip")
Fixes: 5cb57a1aff755 ("KVM: arm64: Zero ID_AA64PFR0_EL1.GIC when no GICv3 is presented to the guest")
Closes: https://lore.kernel.org/r/aRHf6x5umkTYhYJ3@finisterre.sirena.org.uk
Reported-by: Mark Brown <broonie@kernel.org>
Tested-by: Mark Brown <broonie@kernel.org>
Link: https://patch.msgid.link/20251110173010.1918424-1-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 8ae2bca81614861ed00efff6d5320bb061857acd..ec3fbe0b8d525bdc8ab8782526e109f7be518edd 100644 (file)
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -5609,7 +5609,11 @@ int kvm_finalize_sys_regs(struct kvm_vcpu *vcpu)
 
        guard(mutex)(&kvm->arch.config_lock);
 
-       if (!irqchip_in_kernel(kvm)) {
+       /*
+        * This hacks into the ID registers, so only perform it when the
+        * first vcpu runs, or the kvm_set_vm_id_reg() helper will scream.
+        */
+       if (!irqchip_in_kernel(kvm) && !kvm_vm_has_ran_once(kvm)) {
                u64 val;
 
                val = kvm_read_vm_id_reg(kvm, SYS_ID_AA64PFR0_EL1) & ~ID_AA64PFR0_EL1_GIC;