KVM: TDX: Reject fully in-kernel irqchip if EOIs are protected, i.e. for TDX VMs

Reject KVM_CREATE_IRQCHIP if the VM type has protected EOIs, i.e. if KVM
can't intercept EOI and thus can't faithfully emulate level-triggered
interrupts that are routed through the I/O APIC.  For TDX VMs, the
TDX-Module owns the VMX EOI-bitmap and configures all IRQ vectors to have
the CPU accelerate EOIs, i.e. doesn't allow KVM to intercept any EOIs.

KVM already requires a split irqchip[1], but does so during vCPU creation,
which is both too late to allow userspace to fallback to a split irqchip
and a less-than-stellar experience for userspace since an -EINVAL on
KVM_VCPU_CREATE is far harder to debug/triage than failure exactly on
KVM_CREATE_IRQCHIP.  And of course, allowing an action that ultimately
fails is arguably a bug regardless of the impact on userspace.

Link: https://lore.kernel.org/lkml/20250222014757.897978-11-binbin.wu@linux.intel.com [1]
Link: https://lore.kernel.org/lkml/aK3vZ5HuKKeFuuM4@google.com
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sagi Shahar <sagis@google.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Acked-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20250827011726.2451115-1-sagis@google.com
[sean: massage shortlog+changelog, relocate setting has_protected_eoi]
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sagishah

arch/x86/include/asm/kvm_host.h

@@ -1362,6 +1362,7 @@ struct kvm_arch {
 	u8 vm_type;
 	bool has_private_mem;
 	bool has_protected_state;
+	bool has_protected_eoi;
 	bool pre_fault_allowed;
 	struct hlist_head *mmu_page_hash;
 	struct list_head active_mmu_pages;

arch/x86/kvm/vmx/tdx.c

@@ -629,6 +629,11 @@ int tdx_vm_init(struct kvm *kvm)
 	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
 
 	kvm->arch.has_protected_state = true;
+	/*
+	 * TDX Module doesn't allow the hypervisor to modify the EOI-bitmap,
+	 * i.e. all EOIs are accelerated and never trigger exits.
+	 */
+	kvm->arch.has_protected_eoi = true;
 	kvm->arch.has_private_mem = true;
 	kvm->arch.disabled_quirks |= KVM_X86_QUIRK_IGNORE_GUEST_PAT;
 

arch/x86/kvm/x86.c

@@ -6989,6 +6989,15 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
 		if (irqchip_in_kernel(kvm))
 			goto create_irqchip_unlock;
 
+		/*
+		 * Disallow an in-kernel I/O APIC if the VM has protected EOIs,
+		 * i.e. if KVM can't intercept EOIs and thus can't properly
+		 * emulate level-triggered interrupts.
+		 */
+		r = -ENOTTY;
+		if (kvm->arch.has_protected_eoi)
+			goto create_irqchip_unlock;
+
 		r = -EINVAL;
 		if (kvm->created_vcpus)
 			goto create_irqchip_unlock;