linux-kernel - Re: [PATCH] Revert "KVM: x86: Unconditionally enable irqs in guest context"

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Message-ID: <874kjuidgp.fsf@vitty.brq.redhat.com>
Date:   Wed, 06 Jan 2021 11:09:26 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Nitesh Narayan Lal <nitesh@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        seanjc@...gle.com, w90p710@...il.com, pbonzini@...hat.com,
        nitesh@...hat.com, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] Revert "KVM: x86: Unconditionally enable irqs in guest
 context"

Nitesh Narayan Lal <nitesh@...hat.com> writes:

> This reverts commit d7a08882a0a4b4e176691331ee3f492996579534.
>
> After the introduction of the patch:
>
> 	87fa7f3e9: x86/kvm: Move context tracking where it belongs
>
> since we have moved guest_exit_irqoff closer to the VM-Exit, explicit
> enabling of irqs to process pending interrupts should not be required
> within vcpu_enter_guest anymore.
>
> Conflicts:
> 	arch/x86/kvm/svm.c
>
> Signed-off-by: Nitesh Narayan Lal <nitesh@...hat.com>
> ---
>  arch/x86/kvm/svm/svm.c |  9 +++++++++
>  arch/x86/kvm/x86.c     | 11 -----------
>  2 files changed, 9 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index cce0143a6f80..c9b2fbb32484 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4187,6 +4187,15 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,
>  
>  static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu)
>  {
> +	kvm_before_interrupt(vcpu);
> +	local_irq_enable();
> +	/*
> +	 * We must have an instruction with interrupts enabled, so
> +	 * the timer interrupt isn't delayed by the interrupt shadow.
> +	 */
> +	asm("nop");
> +	local_irq_disable();
> +	kvm_after_interrupt(vcpu);
>  }
>  
>  static void svm_sched_in(struct kvm_vcpu *vcpu, int cpu)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3f7c1fc7a3ce..3e17c9ffcad8 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9023,18 +9023,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  
>  	kvm_x86_ops.handle_exit_irqoff(vcpu);
>  
> -	/*
> -	 * Consume any pending interrupts, including the possible source of
> -	 * VM-Exit on SVM 

I kind of liked this part of the comment, the new (old) one in
svm_handle_exit_irqoff() doesn't actually explain what's going on.

> and any ticks that occur between VM-Exit and now.

Looking back, I don't quite understand why we wanted to account ticks
between vmexit and exiting guest context as 'guest' in the first place;
to my understanging 'guest time' is time spent within VMX non-root
operation, the rest is KVM overhead (system). It seems to match how the
accounting is done nowadays after Tglx's 87fa7f3e98a1 ("x86/kvm: Move
context tracking where it belongs").

> -	 * An instruction is required after local_irq_enable() to fully unblock
> -	 * interrupts on processors that implement an interrupt shadow, the
> -	 * stat.exits increment will do nicely.
> -	 */
> -	kvm_before_interrupt(vcpu);
> -	local_irq_enable();
>  	++vcpu->stat.exits;
> -	local_irq_disable();
> -	kvm_after_interrupt(vcpu);
>  
>  	if (lapic_in_kernel(vcpu)) {
>  		s64 delta = vcpu->arch.apic->lapic_timer.advance_expire_delta;

FWIW,

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly