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SubjectRe: [RFC PATCH] KVM: x86: Set PF_VCPU when processing IRQs to fix tick-based accounting
Date
Sean Christopherson <seanjc@google.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@vger.kernel.org
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Sean Christopherson <seanjc@google.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

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