Re: [patch 1/2] KVM: x86: handle double and triple faults for every exception

[Gleb Natapov <gleb@redhat.com>]

On Wed, Nov 11, 2009 at 05:29:48PM -0200, Marcelo Tosatti wrote:
> From: Joerg Roedel <joerg.roedel@amd.com>
> 
> The current KVM x86 exception code handles double and triple faults only for
> page fault exceptions. This patch extends this detection for every exception
> that gets queued for the guest.
> 
> Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
> CC: Jan Kiszka <jan.kiszka@web.de>
> 
> Index: kvm/arch/x86/kvm/x86.c
> ===================================================================
> --- kvm.orig/arch/x86/kvm/x86.c
> +++ kvm/arch/x86/kvm/x86.c
> @@ -170,9 +170,21 @@ void kvm_set_apic_base(struct kvm_vcpu *
>  }
>  EXPORT_SYMBOL_GPL(kvm_set_apic_base);
>  
> +static void handle_multiple_faults(struct kvm_vcpu *vcpu)
> +{
> +       if (vcpu->arch.exception.nr != DF_VECTOR) {
> +               vcpu->arch.exception.nr = DF_VECTOR;
> +               vcpu->arch.exception.error_code = 0;
> +       } else
> +               set_bit(KVM_REQ_TRIPLE_FAULT, &vcpu->requests);
> +}
> +
Making #DF from two bening exceptions is very wrong.

>  void kvm_queue_exception(struct kvm_vcpu *vcpu, unsigned nr)
>  {
> -	WARN_ON(vcpu->arch.exception.pending);
> +	if (vcpu->arch.exception.pending) {
> +		handle_multiple_faults(vcpu);
> +		return;
> +	}
>  	vcpu->arch.exception.pending = true;
>  	vcpu->arch.exception.has_error_code = false;
>  	vcpu->arch.exception.nr = nr;
> @@ -184,24 +196,6 @@ void kvm_inject_page_fault(struct kvm_vc
>  {
>  	++vcpu->stat.pf_guest;
>  
> -	if (vcpu->arch.exception.pending) {
> -		switch(vcpu->arch.exception.nr) {
> -		case DF_VECTOR:
> -			/* triple fault -> shutdown */
> -			set_bit(KVM_REQ_TRIPLE_FAULT, &vcpu->requests);
> -			return;
> -		case PF_VECTOR:
> -			vcpu->arch.exception.nr = DF_VECTOR;
> -			vcpu->arch.exception.error_code = 0;
> -			return;
> -		default:
> -			/* replace previous exception with a new one in a hope
> -			   that instruction re-execution will regenerate lost
> -			   exception */
> -			vcpu->arch.exception.pending = false;
> -			break;
When exceptions are handled serially previous exception have to be
replaced by new one. Think about #PF during #DE. #PF should be handled first
before #DE can proceed.

> -		}
> -	}
>  	vcpu->arch.cr2 = addr;
>  	kvm_queue_exception_e(vcpu, PF_VECTOR, error_code);
>  }
> @@ -214,7 +208,10 @@ EXPORT_SYMBOL_GPL(kvm_inject_nmi);
>  
>  void kvm_queue_exception_e(struct kvm_vcpu *vcpu, unsigned nr, u32 error_code)
>  {
> -	WARN_ON(vcpu->arch.exception.pending);
> +	if (vcpu->arch.exception.pending) {
> +		handle_multiple_faults(vcpu);
> +		return;
> +	}
>  	vcpu->arch.exception.pending = true;
>  	vcpu->arch.exception.has_error_code = true;
>  	vcpu->arch.exception.nr = nr;
> 
> -- 

--
			Gleb.