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

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Date:   Tue, 5 Jan 2021 16:47:03 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Nitesh Narayan Lal <nitesh@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        w90p710@...il.com, pbonzini@...hat.com, vkuznets@...hat.com,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] Revert "KVM: x86: Unconditionally enable irqs in guest
 context"

+tglx

On Tue, Jan 05, 2021, Nitesh Narayan Lal wrote:
> 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.

Ugh, except that commit completely broke tick-based accounting, on both Intel
and AMD.  With guest_exit_irqoff() being called immediately after VM-Exit, any
tick that happens after IRQs are disabled will be accounted to the host.  E.g.
on Intel, even an IRQ VM-Exit that has already been acked by the CPU isn't
processed until kvm_x86_ops.handle_exit_irqoff(), well after PF_VCPU has been
cleared.

CONFIG_VIRT_CPU_ACCOUNTING_GEN=y should still work (I didn't bother to verify).

Thomas, any clever ideas?  Handling IRQs in {vmx,svm}_vcpu_enter_exit() isn't an
option as KVM hasn't restored enough state to handle an IRQ, e.g. PKRU and XCR0
are still guest values.  Is it too heinous to fudge PF_VCPU across KVM's
"pending" IRQ handling?  E.g. this god-awful hack fixes the accounting:

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 836912b42030..5a777fd35b4b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9028,6 +9028,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
        vcpu->mode = OUTSIDE_GUEST_MODE;
        smp_wmb();
 
+       current->flags |= PF_VCPU;
        kvm_x86_ops.handle_exit_irqoff(vcpu);
 
        /*
@@ -9042,6 +9043,7 @@ 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;

> 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 and any ticks that occur between VM-Exit and now.
> -	 * 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;
> -- 
> 2.27.0
>