Re: [PATCH v2 2/4] KVM: VMX: Handle #MCs on VM-Enter/TD-Enter outside of the fastpath

[Binbin Wu]

On 11/19/2025 6:23 AM, Sean Christopherson wrote:
> Handle Machine Checks (#MC) that happen on VM-Enter (VMX or TDX) outside
> of KVM's fastpath so that as much host state as possible is re-loaded
> before invoking the kernel's #MC handler.  The only requirement is that
> KVM invokes the #MC handler before enabling IRQs (and even that could
> _probably_ be related to handling #MCs before enabling preemption).
>
> Waiting to handle #MCs until "more" host state is loaded hardens KVM
> against flaws in the #MC handler, which has historically been quite
> brittle. E.g. prior to commit 5567d11c21a1 ("x86/mce: Send #MC singal from
> task work"), the #MC code could trigger a schedule() with IRQs and
> preemption disabled.  That led to a KVM hack-a-fix in commit 1811d979c716
> ("x86/kvm: move kvm_load/put_guest_xcr0 into atomic context").
>
> Note, vmx_handle_exit_irqoff() is common to VMX and TDX guests.
>
> Cc: Tony Lindgren <tony.lindgren@xxxxxxxxxxxxxxx>
> Cc: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Cc: Jon Kohler <jon@xxxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   arch/x86/kvm/vmx/tdx.c |  3 ---
>   arch/x86/kvm/vmx/vmx.c | 16 +++++++++++-----
>   2 files changed, 11 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index e6105a527372..2d7a4d52ccfb 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -1110,9 +1110,6 @@ fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
>   	if (unlikely((tdx->vp_enter_ret & TDX_SW_ERROR) == TDX_SW_ERROR))
>   		return EXIT_FASTPATH_NONE;
>   
> -	if (unlikely(vmx_get_exit_reason(vcpu).basic == EXIT_REASON_MCE_DURING_VMENTRY))
> -		kvm_machine_check();
> -
>   	trace_kvm_exit(vcpu, KVM_ISA_VMX);
>   
>   	if (unlikely(tdx_failed_vmentry(vcpu)))
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index fdcc519348cd..f369c499b2c3 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7062,10 +7062,19 @@ void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu)

I think the bug in v1 is because the fact that the function is the common path
for both VMX and TDX is overlooked. Do you think it is worth a comment to
tell the function is the common path for both VMX and TDX?

Otherwise,
Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>


>   	if (to_vt(vcpu)->emulation_required)
>   		return;
>   
> -	if (vmx_get_exit_reason(vcpu).basic == EXIT_REASON_EXTERNAL_INTERRUPT)
> +	switch (vmx_get_exit_reason(vcpu).basic) {
> +	case EXIT_REASON_EXTERNAL_INTERRUPT:
>   		handle_external_interrupt_irqoff(vcpu, vmx_get_intr_info(vcpu));
> -	else if (vmx_get_exit_reason(vcpu).basic == EXIT_REASON_EXCEPTION_NMI)
> +		break;
> +	case EXIT_REASON_EXCEPTION_NMI:
>   		handle_exception_irqoff(vcpu, vmx_get_intr_info(vcpu));
> +		break;
> +	case EXIT_REASON_MCE_DURING_VMENTRY:
> +		kvm_machine_check();
> +		break;
> +	default:
> +		break;
> +	}
>   }
>   
>   /*
> @@ -7528,9 +7537,6 @@ fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
>   	if (unlikely(vmx->fail))
>   		return EXIT_FASTPATH_NONE;
>   
> -	if (unlikely((u16)vmx_get_exit_reason(vcpu).basic == EXIT_REASON_MCE_DURING_VMENTRY))
> -		kvm_machine_check();
> -
>   	trace_kvm_exit(vcpu, KVM_ISA_VMX);
>   
>   	if (unlikely(vmx_get_exit_reason(vcpu).failed_vmentry))