linux-kernel - Re: [PATCH v2 11/15] svm: Temporary deactivate AVIC during ExtINT handling

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Message-ID: <82C8A08D-6CB3-4268-BF79-802E1015E365@amazon.de>
Date:   Wed, 28 Aug 2019 19:37:24 +0000
From:   "Graf (AWS), Alexander" <graf@...zon.de>
To:     "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "Schoenherr, Jan H." <jschoenh@...zon.de>,
        "Raslan, KarimAllah" <karahmed@...zon.de>,
        "Lukaszewicz, Rimas" <rimasluk@...zon.com>,
        "Grimm, Jon" <Jon.Grimm@....com>
Subject: Re: [PATCH v2 11/15] svm: Temporary deactivate AVIC during ExtINT
 handling



> Am 28.08.2019 um 17:19 schrieb Suthikulpanit, Suravee <Suravee.Suthikulpanit@....com>:
> 
> Alex,
> 
>> On 8/19/19 5:35 AM, Alexander Graf wrote:
>> 
>> 
>>> On 15.08.19 18:25, Suthikulpanit, Suravee wrote:
>>> AMD AVIC does not support ExtINT. Therefore, AVIC must be temporary
>>> deactivated and fall back to using legacy interrupt injection via vINTR
>>> and interrupt window.
>>> 
>>> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
>>> ---
>>>   arch/x86/kvm/svm.c | 49 
>>> +++++++++++++++++++++++++++++++++++++++++++++----
>>>   1 file changed, 45 insertions(+), 4 deletions(-)
>>> 
>>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>>> index cfa4b13..4690351 100644
>>> --- a/arch/x86/kvm/svm.c
>>> +++ b/arch/x86/kvm/svm.c
>>> @@ -384,6 +384,7 @@ struct amd_svm_iommu_ir {
>>>   static void svm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0);
>>>   static void svm_flush_tlb(struct kvm_vcpu *vcpu, bool invalidate_gpa);
>>>   static void svm_complete_interrupts(struct vcpu_svm *svm);
>>> +static void svm_request_activate_avic(struct kvm_vcpu *vcpu);
>>>   static bool svm_get_enable_apicv(struct kvm *kvm);
>>>   static inline void avic_post_state_restore(struct kvm_vcpu *vcpu);
>>> @@ -4494,6 +4495,15 @@ static int interrupt_window_interception(struct 
>>> vcpu_svm *svm)
>>>   {
>>>       kvm_make_request(KVM_REQ_EVENT, &svm->vcpu);
>>>       svm_clear_vintr(svm);
>>> +
>>> +    /*
>>> +     * For AVIC, the only reason to end up here is ExtINTs.
>>> +     * In this case AVIC was temporarily disabled for
>>> +     * requesting the IRQ window and we have to re-enable it.
>>> +     */
>>> +    if (svm_get_enable_apicv(svm->vcpu.kvm))
>>> +        svm_request_activate_avic(&svm->vcpu);
>> 
>> Would it make sense to add a trace point here and to the other call 
>> sites, so that it becomes obvious in a trace when and why exactly avic 
>> was active/inactive?
>> 
>> The trace point could add additional information on the why.
> 
> Sure, sounds good.
> 
>>> ....
>>> @@ -5522,9 +5558,6 @@ static void enable_irq_window(struct kvm_vcpu 
>>> *vcpu)
>>>   {
>>>       struct vcpu_svm *svm = to_svm(vcpu);
>>> -    if (kvm_vcpu_apicv_active(vcpu))
>>> -        return;
>>> -
>>>       /*
>>>        * In case GIF=0 we can't rely on the CPU to tell us when GIF 
>>> becomes
>>>        * 1, because that's a separate STGI/VMRUN intercept.  The next 
>>> time we
>>> @@ -5534,6 +5567,14 @@ static void enable_irq_window(struct kvm_vcpu 
>>> *vcpu)
>>>        * window under the assumption that the hardware will set the GIF.
>>>        */
>>>       if ((vgif_enabled(svm) || gif_set(svm)) && nested_svm_intr(svm)) {
>>> +        /*
>>> +         * IRQ window is not needed when AVIC is enabled,
>>> +         * unless we have pending ExtINT since it cannot be injected
>>> +         * via AVIC. In such case, we need to temporarily disable AVIC,
>>> +         * and fallback to injecting IRQ via V_IRQ.
>>> +         */
>>> +        if (kvm_vcpu_apicv_active(vcpu))
>>> +            svm_request_deactivate_avic(&svm->vcpu);
>> 
>> Did you test AVIC with nesting? Did you actually run across this issue 
>> there?
> 
> Currently, we have not claimed that AVIC is supported w/ nested VM. 
> That's why we have not enabled AVIC by default yet. We will be looking 
> more into that next.

If it's not supported, please suspend it when we enter a nested guest then? In that case, the above change is also unnecessary, as it only affects nested guests, no?

Alex

> 
> Suravee



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