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

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Message-ID: <e2ecdb77-a6ec-c73f-db66-a9eb4ca1dffd@redhat.com>
Date:   Tue, 5 Jan 2021 20:35:26 -0500
From:   Nitesh Narayan Lal <nitesh@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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"


On 1/5/21 7:47 PM, Sean Christopherson wrote:
> +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.

I did notice some discrepancies in the system time reported after the
introduction of this patch but I wrongly concluded that the behavior is correct.

I reported this yesterday [1] but I think I added your old email ID in
that thread (sorry about that).

>   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.

Right that also explains the higher system time reported by the cpuacct.stats.

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

For the cpuacct stats that I have shared in the other thread, this config was
enabled.

>
> 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;
>

I can give this a try.
What is the right way to test this (via cpuacct stats maybe)?

[1] https://lore.kernel.org/lkml/12a1b9d4-8534-e23a-6bbd-736474928e6b@redhat.com/

-- 
Thanks
Nitesh