From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@redhat.com>
Subject: Re: [PATCH] KVM: prevent spurious exit to userspace during task switch
 emulation.
Date: Thu, 15 Apr 2010 12:36:22 +0300
Message-ID: <4BC6DE16.8040706@redhat.com>
References: <20100415092810.GH18132@redhat.com>
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Cc: kvm@vger.kernel.org, mtosatti@redhat.com
To: Gleb Natapov <gleb@redhat.com>
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On 04/15/2010 12:28 PM, Gleb Natapov wrote:
>
> kvm_task_switch() never requires userspace exit, so no matter what the
> function returns we should not exit to userspace.
>
> Signed-off-by: Gleb Natapov<gleb@redhat.com>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index c773a46..1bd434b 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -2254,7 +2254,8 @@ static int task_switch_interception(struct vcpu_svm *svm)
>   	     (int_vec == OF_VECTOR || int_vec == BP_VECTOR)))
>   		skip_emulated_instruction(&svm->vcpu);
>
> -	return kvm_task_switch(&svm->vcpu, tss_selector, reason);
> +	kvm_task_switch(&svm->vcpu, tss_selector, reason);
> +	return 1;
>   }
>
>   static int cpuid_interception(struct vcpu_svm *svm)
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 453f080..3e1607d 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -3306,8 +3306,7 @@ static int handle_task_switch(struct kvm_vcpu *vcpu)
>   		       type != INTR_TYPE_NMI_INTR))
>   		skip_emulated_instruction(vcpu);
>
> -	if (!kvm_task_switch(vcpu, tss_selector, reason))
> -		return 0;
> +	kvm_task_switch(vcpu, tss_selector, reason);
>
>    

Ignoring the return seems wrong.  At the very least log errors in dmesg 
(rate-limited).

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.