linux-kernel - Re: [PATCH] KVM: x86: optimize local IRQ disable time before vmentry

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Date:   Thu, 9 Mar 2017 13:33:26 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH] KVM: x86: optimize local IRQ disable time before vmentry



On 09/03/2017 13:16, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> Commit b95234c84 (kvm: x86: do not use KVM_REQ_EVENT for APICv interrupt 
> injection) disables interrupts before setting vcpu->mode to fix an race:
> 
> | The IPI for posted interrupts may be issued between setting vcpu->mode = 
> | IN_GUEST_MODE and disabling interrupts. If that happens, 
> | kvm_trigger_posted_interrupt returns true, but smp_kvm_posted_intr_ipi 
> | doesn't do anything about it.  The guest is entered with PIR.ON, but the 
> | posted interrupt IPI has not been sent and the interrupt is only delivered 
> | to the guest on the next vmentry (if any).
> 
> However the race has already been fixed by the side-effects of moving the 
> RVI update after IN_GUEST_MODE in the commit:
> 
> - The IPI for posted interrupts is issued after setting vcpu->mode = 
>   IN_GUEST_MODE and disabling interrupts, the posted interrupt IPI is 
>   delayed until the guest enters non-root mode; it is then trapped by 
>   the processor causing the interrupt to be injected.
> - The IPI for posted interrupts is issued between setting vcpu->mode = 
>   IN_GUEST_MODE and disabling interrupts, the movement of the RVI update 
>   after IN_GUEST_MODE in the commit will catch the new PIR, and set RVI.
> 
> This patch tries to reduce the local IRQ disable time by restoring the 
> place of disable local IRQ in order to improve host kernel responsibility 
> to some degree.

This should not make any difference; we're talking of a few dozen clock
cycles, and it's quite possible that the extra chance of getting a PI
interrupt in the host negates it.

Paolo

> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
>  arch/x86/kvm/x86.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 1faf620..9d97041 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6857,12 +6857,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	kvm_x86_ops->prepare_guest_switch(vcpu);
>  	kvm_load_guest_fpu(vcpu);
>  
> -	/*
> -	 * Disable IRQs before setting IN_GUEST_MODE.  Posted interrupt
> -	 * IPI are then delayed after guest entry, which ensures that they
> -	 * result in virtual interrupt delivery.
> -	 */
> -	local_irq_disable();
>  	vcpu->mode = IN_GUEST_MODE;
>  
>  	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
> @@ -6881,6 +6875,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	 */
>  	smp_mb__after_srcu_read_unlock();
>  
> +	local_irq_disable();
> +
>  	/*
>  	 * This handles the case where a posted interrupt was
>  	 * notified with kvm_vcpu_kick.
>