linux-kernel - [PATCH] KVM: arm64: gic-v3: Only set ICH

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Message-ID: <20251007160704.1673584-1-sascha.bischoff@arm.com>
Date: Tue, 7 Oct 2025 16:07:13 +0000
From: Sascha Bischoff <Sascha.Bischoff@....com>
To: "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "kvmarm@...ts.linux.dev"
	<kvmarm@...ts.linux.dev>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC: nd <nd@....com>, "maz@...nel.org" <maz@...nel.org>,
	"oliver.upton@...ux.dev" <oliver.upton@...ux.dev>, Joey Gouly
	<Joey.Gouly@....com>, Suzuki Poulose <Suzuki.Poulose@....com>,
	"yuzenghui@...wei.com" <yuzenghui@...wei.com>
Subject: [PATCH] KVM: arm64: gic-v3: Only set ICH_HCR traps for v2-on-v3 or v3
 guests

The ICH_HCR_EL2 traps are used when running on GICv3 hardware, or when
running a GICv3-based guest using FEAT_GCIE_LEGACY on GICv5
hardware. When running a GICv2 guest on GICv3 hardware the traps are
used to ensure that the guest never sees any part of GICv3 (only GICv2
is visible to the guest), and when running a GICv3 guest they are used
to trap in specific scenarios. They are not applicable for a
GICv2-native guest, and won't be applicable for a(n upcoming) GICv5
guest.

The traps themselves are configured in the vGIC CPU IF state, which is
stored as a union. Updating the wrong aperture of the union risks
corrupting state, and therefore needs to be avoided at all costs.

Bail early if we're not running a compatible guest (GICv2 on GICv3
hardware, GICv3 native, GICv3 on GICv5 hardware). Trap everything
unconditionally if we're running a GICv2 guest on GICv3
hardware. Otherwise, conditionally set up GICv3-native trapping.

Signed-off-by: Sascha Bischoff <sascha.bischoff@....com>
---
 arch/arm64/kvm/vgic/vgic-v3.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/vgic/vgic-v3.c b/arch/arm64/kvm/vgic/vgic-v3.c
index b9ad7c42c5b01..bc42c05b53bac 100644
--- a/arch/arm64/kvm/vgic/vgic-v3.c
+++ b/arch/arm64/kvm/vgic/vgic-v3.c
@@ -297,8 +297,11 @@ void vcpu_set_ich_hcr(struct kvm_vcpu *vcpu)
 {
 	struct vgic_v3_cpu_if *vgic_v3 = &vcpu->arch.vgic_cpu.vgic_v3;

+	if (!vgic_is_v3(vcpu->kvm))
+		return;
+
 	/* Hide GICv3 sysreg if necessary */
-	if (!kvm_has_gicv3(vcpu->kvm)) {
+	if (vcpu->kvm->arch.vgic.vgic_model == KVM_DEV_TYPE_ARM_VGIC_V2) {
 		vgic_v3->vgic_hcr |= (ICH_HCR_EL2_TALL0 | ICH_HCR_EL2_TALL1 |
 				      ICH_HCR_EL2_TC);
 		return;
-- 
2.34.1