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Subject: Re: [PATCH RFC] KVM: x86: vmx: throttle immediate exit through
 preemtion timer to assist buggy guests
To:     Vitaly Kuznetsov <vkuznets@redhat.com>, kvm@vger.kernel.org
Cc:     =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Liran Alon <liran.alon@oracle.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        linux-kernel@vger.kernel.org
References: <20190328203110.20655-1-vkuznets@redhat.com>
 <f64742ef-c67d-db0f-d812-2b6f5048d7ee@redhat.com>
 <87d0m93frp.fsf@vitty.brq.redhat.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
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Message-ID: <89d4189b-de6a-7634-de8b-29a044a86e12@redhat.com>
Date:   Fri, 29 Mar 2019 16:01:13 +0100
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On 29/03/19 15:40, Vitaly Kuznetsov wrote:
> Paolo Bonzini <pbonzini@redhat.com> writes:
> 
>> On 28/03/19 21:31, Vitaly Kuznetsov wrote:
>>>
>>> The 'hang' scenario develops like this:
>>> 1) Hyper-V boots and QEMU is trying to inject two irq simultaneously. One
>>>  of them is level-triggered. KVM injects the edge-triggered one and
>>>  requests immediate exit to inject the level-triggered:
>>>
>>>  kvm_set_irq:          gsi 23 level 1 source 0
>>>  kvm_msi_set_irq:      dst 0 vec 80 (Fixed|physical|level)
>>>  kvm_apic_accept_irq:  apicid 0 vec 80 (Fixed|edge)
>>>  kvm_msi_set_irq:      dst 0 vec 96 (Fixed|physical|edge)
>>>  kvm_apic_accept_irq:  apicid 0 vec 96 (Fixed|edge)
>>>  kvm_nested_vmexit_inject: reason EXTERNAL_INTERRUPT info1 0 info2 0 int_info 80000060 int_info_err 0
>>>
>>> 2) Hyper-V requires one of its VMs to run to handle the situation but
>>>  immediate exit happens:
>>>
>>>  kvm_entry:            vcpu 0
>>>  kvm_exit:             reason VMRESUME rip 0xfffff80006a40115 info 0 0
>>>  kvm_entry:            vcpu 0
>>>  kvm_exit:             reason PREEMPTION_TIMER rip 0xfffff8022f3d8350 info 0 0
>>>  kvm_nested_vmexit:    rip fffff8022f3d8350 reason PREEMPTION_TIMER info1 0 info2 0 int_info 0 int_info_err 0
>>>  kvm_nested_vmexit_inject: reason EXTERNAL_INTERRUPT info1 0 info2 0 int_info 80000050 int_info_err 0
>>
>> I supposed before this there was an eoi for vector 96?
> 
> AFAIR: no, it seems that it is actually the VM it is trying to resume
> (Windows partition?) which needs to do some work and with the preemtion
> timer of 0 we don't allow it to.

kvm_apic_accept_irq placed IRQ 96 in IRR, and Hyper-V should be running
with "acknowledge interrupt on exit" since int_info is nonzero in
kvm_nested_vmexit_inject.

Therefore, at the kvm_nested_vmexit_inject tracepoint KVM should have
set bit 96 in ISR; and because PPR is now 96, interrupt 80 should have
never been delivered.  Unless 96 is an auto-EOI interrupt, in which case
this comment would apply

          /*
           * For auto-EOI interrupts, there might be another pending
           * interrupt above PPR, so check whether to raise another
           * KVM_REQ_EVENT.
           */

IIRC there was an enlightenment to tell Windows "I support auto-EOI but
please don't use it".  If this is what's happening, that would also fix it.

Thanks,

Paolo