Re: [PATCH 1/2] KVM: Fix race between nmi injection and enabling nmi window

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-02-03 16:02, Avi Kivity wrote:
> The interrupt injection logic looks something like
> 
>   if an nmi is pending, and nmi injection allowed
>     inject nmi
>   if an nmi is pending
>     request exit on nmi window
> 
> the problem is that "nmi is pending" can be set asynchronously by
> the PIT; if it happens to fire between the two if statements, we
> will request an nmi window even though nmi injection is allowed.  On
> SVM, this has disasterous results, since it causes eflags.TF to be
> set in random guest code.

Good point. Fortunately never seen on production machines so far here
(we have very moderate NMI rates).

> 
> The fix is simple; make nmi_pending asynchronous using the standard

You mean synchronous, no?

> vcpu->requests mechanism; this ensures the code above is completely
> synchronous wrt nmi_pending.
> 
> Signed-off-by: Avi Kivity <avi@redhat.com>
> ---
>  arch/x86/kvm/x86.c       |    4 +++-
>  include/linux/kvm_host.h |    1 +
>  2 files changed, 4 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a7f65aa..abe76c0 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -360,8 +360,8 @@ void kvm_propagate_fault(struct kvm_vcpu *vcpu, struct x86_exception *fault)
>  
>  void kvm_inject_nmi(struct kvm_vcpu *vcpu)
>  {
> +	kvm_make_request(KVM_REQ_NMI, vcpu);
>  	kvm_make_request(KVM_REQ_EVENT, vcpu);
> -	vcpu->arch.nmi_pending = 1;
>  }
>  EXPORT_SYMBOL_GPL(kvm_inject_nmi);
>  
> @@ -5182,6 +5182,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  			r = 1;
>  			goto out;
>  		}
> +		if (kvm_check_request(KVM_REQ_NMI, vcpu))
> +			vcpu->arch.nmi_pending = true;
>  	}
>  
>  	r = kvm_mmu_reload(vcpu);
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index c8dee22..7581090 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -43,6 +43,7 @@
>  #define KVM_REQ_DEACTIVATE_FPU    10
>  #define KVM_REQ_EVENT             11
>  #define KVM_REQ_APF_HALT          12
> +#define KVM_REQ_NMI               13
>  
>  #define KVM_USERSPACE_IRQ_SOURCE_ID	0
>  

Looks good.

Jan

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