[PATCH v1 2/4] KVM: arm64: Clear HCR_EL2.RW for 32-bit non-protected vCPUs

[Fuad Tabba <fuad.tabba@linux.dev>]

In pKVM, KVM_RUN on a vCPU created with KVM_ARM_VCPU_EL1_32BIT fails
with KVM_EXIT_FAIL_ENTRY. pkvm_vcpu_reset_hcr(), pKVM's EL2 counterpart
of vcpu_set_hcr(), never clears HCR_EL2.RW, so the first ERET into the
vCPU is an illegal exception return. Protected VMs are AArch64-only, so
only non-protected VMs are affected.

Clear RW for 32-bit vCPUs. The vCPU's features come from the host, and
on a CPU without AArch32 EL1 a cleared RW would make EL2 switch
registers that are UNDEFINED there. Clear it only when the system has
AArch32 EL1, the same check system_supported_vcpu_features() makes on
the host.

Fixes: b56680de9c648 ("KVM: arm64: Initialize trap register values in hyp in pKVM")
Cc: stable@vger.kernel.org
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
 arch/arm64/kvm/hyp/nvhe/pkvm.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
index 459bd9eb7e4bc..affc9595fda20 100644
--- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
+++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
@@ -54,6 +54,14 @@ static void pkvm_vcpu_reset_hcr(struct kvm_vcpu *vcpu)
 	else
 		vcpu->arch.hcr_el2 |= HCR_TID2;
 
+	/*
+	 * At EL2, vcpu_el1_is_32bit() reads HCR_EL2.RW, and EL2 switches the
+	 * *32_EL2 registers when it returns true; they're UNDEFINED without AArch32 EL1.
+	 */
+	if (vcpu_has_feature(vcpu, KVM_ARM_VCPU_EL1_32BIT) &&
+	    cpus_have_final_cap(ARM64_HAS_32BIT_EL1))
+		vcpu->arch.hcr_el2 &= ~HCR_RW;
+
 	if (vcpu_has_ptrauth(vcpu))
 		vcpu->arch.hcr_el2 |= (HCR_API | HCR_APK);
 
-- 
2.39.5