Re: [PATCH] KVM: TDX: Force split irqchip for TDX at irqchip creation time

[Binbin Wu <binbin.wu@linux.intel.com>]

On 8/27/2025 5:34 AM, Sagi Shahar wrote:
> TDX module protects the EOI-bitmap which prevents the use of in-kernel
> I/O APIC. See more details in the original patch [1]
>
> The current implementation already enforces the use of split irqchip for
> TDX but it does so at the vCPU creation time which is generally to late
                                                                 ^
                                                                 too
> to fallback to split irqchip.
>
> This patch follows Sean's recomendation from [2] and move the check if

recomendation -> recommendation

> I/O APIC is supported for the VM at irqchip creation time.
>
> [1] https://lore.kernel.org/lkml/20250222014757.897978-11-binbin.wu@linux.intel.com/
> [2] https://lore.kernel.org/lkml/aK3vZ5HuKKeFuuM4@google.com/
>
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Sagi Shahar <sagis@google.com>
> ---
>   arch/x86/include/asm/kvm_host.h |  3 +++
>   arch/x86/kvm/vmx/tdx.c          | 15 ++++++++-------
>   arch/x86/kvm/x86.c              | 10 ++++++++++
>   3 files changed, 21 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index f19a76d3ca0e..cb22fc48cdec 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1357,6 +1357,7 @@ 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;
> @@ -2284,6 +2285,8 @@ void kvm_configure_mmu(bool enable_tdp, int tdp_forced_root_level,
>   
>   #define kvm_arch_has_readonly_mem(kvm) (!(kvm)->arch.has_protected_state)
>   
> +#define kvm_arch_has_protected_eoi(kvm) (!(kvm)->arch.has_protected_eoi)
> +
>   static inline u16 kvm_read_ldt(void)
>   {
>   	u16 ldt;
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 66744f5768c8..8c270a159692 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -658,6 +658,12 @@ int tdx_vm_init(struct kvm *kvm)
>   	 */
>   	kvm->max_vcpus = min_t(int, kvm->max_vcpus, num_present_cpus());
>   
> +	/*
> +	 * 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_tdx->state = TD_STATE_UNINITIALIZED;
>   
>   	return 0;
> @@ -671,13 +677,8 @@ int tdx_vcpu_create(struct kvm_vcpu *vcpu)
>   	if (kvm_tdx->state != TD_STATE_INITIALIZED)
>   		return -EIO;
>   
> -	/*
> -	 * TDX module mandates APICv, which requires an in-kernel local APIC.
> -	 * Disallow an in-kernel I/O APIC, because level-triggered interrupts
> -	 * and thus the I/O APIC as a whole can't be faithfully emulated in KVM.
> -	 */
> -	if (!irqchip_split(vcpu->kvm))
> -		return -EINVAL;
> +	/* Split irqchip should be enforced at irqchip creation time. */
> +	KVM_BUG_ON(irqchip_split(vcpu->kvm), vcpu->kvm);

Should be
KVM_BUG_ON(!irqchip_split(vcpu->kvm), vcpu->kvm);

>   
>   	fpstate_set_confidential(&vcpu->arch.guest_fpu);
>   	vcpu->arch.apic->guest_apic_protected = true;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a1c49bc681c4..a846dd3dcb23 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6966,6 +6966,16 @@ 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 for platforms that has protected
> +		 * EOI (such as TDX). The hypervisor can't modify the EOI-bitmap
> +		 * on these platforms which prevents the proper emulation of
> +		 * level-triggered interrupts.
> +		 */
> +		r = -ENOTTY;
> +		if (kvm_arch_has_protected_eoi(kvm))
> +			goto create_irqchip_unlock;
> +
>   		r = -EINVAL;
>   		if (kvm->created_vcpus)
>   			goto create_irqchip_unlock;