Re: [RESEND PATCH 5/6] KVM: x86/VMX: add kvm_vmx_reinject_nmi_irq() for NMI/IRQ reinjection

[Peter Zijlstra]

On Wed, Nov 09, 2022 at 10:15:44PM -0800, Xin Li wrote:
> To eliminate dispatching NMI/IRQ through the IDT, add
> kvm_vmx_reinject_nmi_irq(), which calls external_interrupt()
> for IRQ reinjection.
> 
> Lastly replace calling a NMI/IRQ handler in an IDT descriptor
> with calling kvm_vmx_reinject_nmi_irq().
> 
> Signed-off-by: H. Peter Anvin (Intel) <hpa@xxxxxxxxx>
> Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>

Idem.


> +#if IS_ENABLED(CONFIG_KVM_INTEL)
> +/*
> + * KVM VMX reinjects NMI/IRQ on its current stack, it's a sync
> + * call thus the values in the pt_regs structure are not used in
> + * executing NMI/IRQ handlers, except cs.RPL and flags.IF, which
> + * are both always 0 in the VMX NMI/IRQ reinjection context. Thus
> + * we simply allocate a zeroed pt_regs structure on current stack
> + * to call external_interrupt().
> + */
> +void kvm_vmx_reinject_nmi_irq(u32 vector)

noinstr ?

> +{
> +	struct pt_regs irq_regs;
> +
> +	memset(&irq_regs, 0, sizeof(irq_regs));
> +
> +	if (vector == NMI_VECTOR)
> +		return exc_nmi(&irq_regs);
> +
> +	external_interrupt(&irq_regs, vector);
> +}
> +EXPORT_SYMBOL_GPL(kvm_vmx_reinject_nmi_irq);
> +#endif

> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 63247c57c72c..b457e4888468 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -46,6 +46,7 @@
>  #include <asm/mshyperv.h>
>  #include <asm/mwait.h>
>  #include <asm/spec-ctrl.h>
> +#include <asm/traps.h>
>  #include <asm/virtext.h>
>  #include <asm/vmx.h>
>  
> @@ -6758,15 +6759,11 @@ static void vmx_apicv_post_state_restore(struct kvm_vcpu *vcpu)
>  	memset(vmx->pi_desc.pir, 0, sizeof(vmx->pi_desc.pir));
>  }
>  
> -void vmx_do_interrupt_nmi_irqoff(unsigned long entry);
> -
> -static void handle_interrupt_nmi_irqoff(struct kvm_vcpu *vcpu,
> -					unsigned long entry)
> +static void handle_interrupt_nmi_irqoff(struct kvm_vcpu *vcpu, u32 vector)
>  {
> -	bool is_nmi = entry == (unsigned long)asm_exc_nmi_noist;
> -
> -	kvm_before_interrupt(vcpu, is_nmi ? KVM_HANDLING_NMI : KVM_HANDLING_IRQ);
> -	vmx_do_interrupt_nmi_irqoff(entry);
> +	kvm_before_interrupt(vcpu, vector == NMI_VECTOR ?
> +				   KVM_HANDLING_NMI : KVM_HANDLING_IRQ);
> +	kvm_vmx_reinject_nmi_irq(vector);
>  	kvm_after_interrupt(vcpu);
>  }
>  
> @@ -6792,7 +6789,6 @@ static void handle_nm_fault_irqoff(struct kvm_vcpu *vcpu)
>  
>  static void handle_exception_nmi_irqoff(struct vcpu_vmx *vmx)
>  {
> -	const unsigned long nmi_entry = (unsigned long)asm_exc_nmi_noist;
>  	u32 intr_info = vmx_get_intr_info(&vmx->vcpu);
>  
>  	/* if exit due to PF check for async PF */
> @@ -6806,20 +6802,19 @@ static void handle_exception_nmi_irqoff(struct vcpu_vmx *vmx)
>  		kvm_machine_check();
>  	/* We need to handle NMIs before interrupts are enabled */
>  	else if (is_nmi(intr_info))
> -		handle_interrupt_nmi_irqoff(&vmx->vcpu, nmi_entry);
> +		handle_interrupt_nmi_irqoff(&vmx->vcpu, NMI_VECTOR);
>  }
>  
>  static void handle_external_interrupt_irqoff(struct kvm_vcpu *vcpu)
>  {
>  	u32 intr_info = vmx_get_intr_info(vcpu);
>  	unsigned int vector = intr_info & INTR_INFO_VECTOR_MASK;
> -	gate_desc *desc = (gate_desc *)host_idt_base + vector;
>  
>  	if (KVM_BUG(!is_external_intr(intr_info), vcpu->kvm,
>  	    "KVM: unexpected VM-Exit interrupt info: 0x%x", intr_info))
>  		return;
>  
> -	handle_interrupt_nmi_irqoff(vcpu, gate_offset(desc));
> +	handle_interrupt_nmi_irqoff(vcpu, vector);
>  	vcpu->arch.at_instruction_boundary = true;
>  }

How does any of this work? You're calling into entry/noinstr code from a
random context.