linux-kernel - Re: [PATCH v3 3/9] KVM: x86: Defer tick-based accounting 'til after IRQ handling

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Date:   Wed, 21 Apr 2021 01:14:02 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Michael Tokarev <mjt@....msk.ru>,
        Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [PATCH v3 3/9] KVM: x86: Defer tick-based accounting 'til after
 IRQ handling

On Thu, Apr 15, 2021 at 03:21:00PM -0700, Sean Christopherson wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> When using tick-based accounting, defer the call to account guest time
> until after servicing any IRQ(s) that happened in the guest or
> immediately after VM-Exit.  Tick-based accounting of vCPU time relies on
> PF_VCPU being set when the tick IRQ handler runs, and IRQs are blocked
> throughout {svm,vmx}_vcpu_enter_exit().
> 
> This fixes a bug[*] where reported guest time remains '0', even when
> running an infinite loop in the guest.
> 
> [*] https://bugzilla.kernel.org/show_bug.cgi?id=209831
> 
> Fixes: 87fa7f3e98a131 ("x86/kvm: Move context tracking where it belongs")
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Sean Christopherson <seanjc@...gle.com>
> Cc: Michael Tokarev <mjt@....msk.ru>
> Cc: stable@...r.kernel.org#v5.9-rc1+
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> Co-developed-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 16fb39503296..e4d475df1d4a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9230,6 +9230,14 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	local_irq_disable();
>  	kvm_after_interrupt(vcpu);
>  
> +	/*
> +	 * When using tick-based accounting, wait until after servicing IRQs to
> +	 * account guest time so that any ticks that occurred while running the
> +	 * guest are properly accounted to the guest.
> +	 */
> +	if (!vtime_accounting_enabled_this_cpu())
> +		vtime_account_guest_exit();

Can we rather have instead:

static inline void tick_account_guest_exit(void)
{
	if (!vtime_accounting_enabled_this_cpu())
		current->flags &= ~PF_VCPU;
}

It duplicates a bit of code but I think this will read less confusing.

Thanks.

> +
>  	if (lapic_in_kernel(vcpu)) {
>  		s64 delta = vcpu->arch.apic->lapic_timer.advance_expire_delta;
>  		if (delta != S64_MIN) {
> -- 
> 2.31.1.368.gbe11c130af-goog
>