linux-kernel - Re: [RFC PATCH] KVM: x86: Set PF_VCPU when processing IRQs to fix tick-based accounting

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Message-ID: <87lfbyonth.fsf@vitty.brq.redhat.com>
Date:   Mon, 08 Feb 2021 11:28:58 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC PATCH] KVM: x86: Set PF_VCPU when processing IRQs to fix
 tick-based accounting

Sean Christopherson <seanjc@...gle.com> writes:

> Temporarily set PF_VCPU while processing IRQ VM-Exits so that a tick IRQ
> accounts the time slice to the guest.  Tick-based accounting of guest
> time is currently broken as PF_VCPU is only set during the relatively
> short VM-Enter sequence, which runs entirely with IRQs disabled, and IRQs
> that occur in the guest are processed well after PF_VCPU is cleared.
>
> Keep PF_VCPU set across both VMX's processing of already-acked IRQs
> (handle_exit_irqoff()) and the explicit IRQ window (SVM's processing,
> plus ticks that occur immediately after VM-Exit on both VMX and SVM).
>
> Fixes: 87fa7f3e98a1 ("x86/kvm: Move context tracking where it belongs")
> Cc: stable@...r.kernel.org
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>
> This is quite obnoxious, hence the RFC, but I can't think of a clever,
> less ugly way to fix the accounting.
>
>  arch/x86/kvm/x86.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d9f931c63293..6ddf341cd755 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9118,6 +9118,13 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	vcpu->mode = OUTSIDE_GUEST_MODE;
>  	smp_wmb();
>  
> +	/*
> +	 * Temporarily pretend this task is running a vCPU when potentially
> +	 * processing an IRQ exit, including the below opening of an IRQ
> +	 * window.  Tick-based accounting of guest time relies on PF_VCPU
> +	 * being set when the tick IRQ handler runs.
> +	 */
> +	current->flags |= PF_VCPU;

Should we do it only when !vtime_accounting_enabled_cpu() maybe?

>  	static_call(kvm_x86_handle_exit_irqoff)(vcpu);
>  
>  	/*
> @@ -9132,6 +9139,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	++vcpu->stat.exits;
>  	local_irq_disable();
>  	kvm_after_interrupt(vcpu);
> +	current->flags &= ~PF_VCPU;
>  
>  	if (lapic_in_kernel(vcpu)) {
>  		s64 delta = vcpu->arch.apic->lapic_timer.advance_expire_delta;

-- 
Vitaly