Re: [PATCH] KVM: x86: move steal time initialization to vcpu entry time

[Luis Henriques <luis.henriques@canonical.com>]

On Fri, Mar 11, 2016 at 04:53:11PM +0800, Liang Chen wrote:
> From: Marcelo Tosatti <mtosatti@redhat.com>
> 
> commit 7cae2bedcbd4680b155999655e49c27b9cf020fa upstream
> 
> This patch is needed on lts 3.4, 3.10, 3.12, 3.14, 3.18, and 4.1 for
> the same issue. Can you please to pick it up?

Thanks, I'm also including this in the 3.16 kernel.

Cheers,
--
Lu�s

> 
> As reported at https://bugs.launchpad.net/qemu/+bug/1494350,
> it is possible to have vcpu->arch.st.last_steal initialized
> from a thread other than vcpu thread, say the iothread, via
> KVM_SET_MSRS.
> 
> Which can cause an overflow later (when subtracting from vcpu threads
> sched_info.run_delay).
> 
> To avoid that, move steal time accumulation to vcpu entry time,
> before copying steal time data to guest.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> Reviewed-by: David Matlack <dmatlack@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/x86.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d6e8bd0..7d4f352 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1987,6 +1987,8 @@ static void accumulate_steal_time(struct kvm_vcpu *vcpu)
>  
>  static void record_steal_time(struct kvm_vcpu *vcpu)
>  {
> +	accumulate_steal_time(vcpu);
> +
>  	if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED))
>  		return;
>  
> @@ -2120,12 +2122,6 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		if (!(data & KVM_MSR_ENABLED))
>  			break;
>  
> -		vcpu->arch.st.last_steal = current->sched_info.run_delay;
> -
> -		preempt_disable();
> -		accumulate_steal_time(vcpu);
> -		preempt_enable();
> -
>  		kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
>  
>  		break;
> @@ -2808,7 +2804,6 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  		vcpu->cpu = cpu;
>  	}
>  
> -	accumulate_steal_time(vcpu);
>  	kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
>  }
>  
> -- 
> 1.9.1
> 
> 
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