[PATCH] KVM: arm64: Finalize ID registers only once per VM

[PATCH] KVM: arm64: Finalize ID registers only once per VM

[Marc Zyngier]

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>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/kvm/sys_regs.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 3bf7005258f07..19afcd833d6fa 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -5624,7 +5624,11 @@ 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;
-- 
2.47.3

Re: [PATCH] KVM: arm64: Finalize ID registers only once per VM

[Mark Brown]

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On Mon, Nov 10, 2025 at 05:30:10PM +0000, Marc Zyngier wrote:
> 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.

This resolves all the test issues I was seeing:

Tested-by: Mark Brown <broonie@kernel.org>

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Re: [PATCH] KVM: arm64: Finalize ID registers only once per VM

[Marc Zyngier]

On Mon, 10 Nov 2025 17:30:10 +0000, Marc Zyngier wrote:
> 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.
> 
> [...]

Applied to fixes, thanks!

[1/1] KVM: arm64: Finalize ID registers only once per VM
      commit: 0f559cd91e37b7978e4198ca2fbf7eb95df11361

Cheers,

	M.
-- 
Without deviation from the norm, progress is not possible.