linux-kernel - Re: [PATCH RFC] KVM: x86: vmx: throttle immediate exit through preemtion timer to assist buggy guests

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Message-ID: <87v9zy15mc.fsf@vitty.brq.redhat.com>
Date:   Mon, 01 Apr 2019 10:39:39 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Liran Alon <liran.alon@...cle.com>
Cc:     kvm@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] KVM: x86: vmx: throttle immediate exit through preemtion timer to assist buggy guests

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 29/03/19 16:32, Liran Alon wrote:
>> Paolo I am not sure this is the case here. Please read my other
>> replies in this email thread.
>> 
>> I think this is just a standard issue of a level-triggered interrupt
>> handler in L1 (Hyper-V) that performs EOI before it lowers the
>> irq-line. I don’t think vector 96 is even related to the issue at
>> hand here. This is why after it was already handled, the loop of
>> EXTERNAL_INTERRUPT happens on vector 80 and not vector 96.
>
> Hmm... Vitaly, what machine were you testing on---does it have APIC-v?
> If not, then you should have seen either an EOI for irq 96 or a TPR
> below threshold vmexit.  However, if it has APIC-v then you wouldn't
> have seen any of this (you only see the EOI for irq 80 because it's
> level triggered) and Liran is probably right.
>

It does, however, the issue is reproducible with and without
it. Moreover, I think the second simultaneous IRQ is just a red herring;
Here is another trace (enable_apicv). Posting it non-stripped and hope
your eyes will catch something I'm missing:

[001] 513675.736316: kvm_exit:             reason VMRESUME rip 0xfffff80002cae115 info 0 0
[001] 513675.736321: kvm_entry:            vcpu 0
[001] 513675.736565: kvm_exit:             reason EXTERNAL_INTERRUPT rip 0xfffff80362dcd26d info 0 800000ec
[001] 513675.736566: kvm_nested_vmexit:    rip fffff80362dcd26d reason EXTERNAL_INTERRUPT info1 0 info2 0 int_info 800000ec int_info_err 0
[001] 513675.736568: kvm_entry:            vcpu 0
[001] 513675.736650: kvm_exit:             reason EPT_VIOLATION rip 0xfffff80362dcd230 info 182 0
[001] 513675.736651: kvm_nested_vmexit:    rip fffff80362dcd230 reason EPT_VIOLATION info1 182 info2 0 int_info 0 int_info_err 0
[001] 513675.736651: kvm_page_fault:       address 261200000 error_code 182

-> injecting

[008] 513675.737059: kvm_set_irq:          gsi 23 level 1 source 0
[008] 513675.737061: kvm_msi_set_irq:      dst 0 vec 80 (Fixed|physical|level)
[008] 513675.737062: kvm_apic_accept_irq:  apicid 0 vec 80 (Fixed|edge)
[001] 513675.737233: kvm_nested_vmexit_inject: reason EXTERNAL_INTERRUPT info1 0 info2 0 int_info 80000050 int_info_err 0
[001] 513675.737239: kvm_entry:            vcpu 0
[001] 513675.737243: kvm_exit:             reason EOI_INDUCED rip 0xfffff80002c85e1a info 50 0

-> immediate EOI causing re-injection (even preemption timer is not
involved here).

[001] 513675.737244: kvm_eoi:              apicid 0 vector 80
[001] 513675.737245: kvm_fpu:              unload
[001] 513675.737246: kvm_userspace_exit:   reason KVM_EXIT_IOAPIC_EOI (26)
[001] 513675.737256: kvm_set_irq:          gsi 23 level 1 source 0
[001] 513675.737259: kvm_msi_set_irq:      dst 0 vec 80 (Fixed|physical|level)
[001] 513675.737260: kvm_apic_accept_irq:  apicid 0 vec 80 (Fixed|edge)
[001] 513675.737264: kvm_fpu:              load
[001] 513675.737265: kvm_entry:            vcpu 0
[001] 513675.737271: kvm_exit:             reason VMRESUME rip 0xfffff80002cae115 info 0 0
[001] 513675.737278: kvm_entry:            vcpu 0
[001] 513675.737282: kvm_exit:             reason PREEMPTION_TIMER rip 0xfffff80362dcc2d0 info 0 0
[001] 513675.737283: kvm_nested_vmexit:    rip fffff80362dcc2d0 reason PREEMPTION_TIMER info1 0 info2 0 int_info 0 int_info_err 0
[001] 513675.737285: kvm_nested_vmexit_inject: reason EXTERNAL_INTERRUPT info1 0 info2 0 int_info 80000050 int_info_err 0
[001] 513675.737289: kvm_entry:            vcpu 0
[001] 513675.737293: kvm_exit:             reason EOI_INDUCED rip 0xfffff80002c85e1a info 50 0
[001] 513675.737293: kvm_eoi:              apicid 0 vector 80
[001] 513675.737294: kvm_fpu:              unload
[001] 513675.737295: kvm_userspace_exit:   reason KVM_EXIT_IOAPIC_EOI (26)
[001] 513675.737299: kvm_set_irq:          gsi 23 level 1 source 0
[001] 513675.737299: kvm_msi_set_irq:      dst 0 vec 80 (Fixed|physical|level)
[001] 513675.737300: kvm_apic_accept_irq:  apicid 0 vec 80 (Fixed|edge)
[001] 513675.737302: kvm_fpu:              load
[001] 513675.737303: kvm_entry:            vcpu 0
[001] 513675.737307: kvm_exit:             reason VMRESUME rip 0xfffff80002cae115 info 0 0

...

-- 
Vitaly