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@xxxxxxxxxx/
--
Thanks
Nitesh
