linux-kernel - Re: [PATCH 6/6] svm: Temporary deactivate AVIC during ExtINT handling

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Message-ID: <5f86b2ac-3a62-ab3d-ea00-59f1aaafa3f1@amd.com>
Date:   Mon, 3 Jun 2019 18:58:33 +0000
From:   "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>
To:     Jan H. Schönherr <jschoenh@...zon.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     "joro@...tes.org" <joro@...tes.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>
Subject: Re: [PATCH 6/6] svm: Temporary deactivate AVIC during ExtINT handling

Hi Jan,

On 5/8/19 12:37 PM, Jan H. Schönherr wrote:
> [CAUTION: External Email]
> 
> Hi Suravee.
> 
> I wonder, how this interacts with Hyper-V SynIC; see comments below.
> 
> On 22/03/2019 12.57, 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.
>>
>> Introduce svm_request_activate/deactivate_avic() helper functions,
>> which handle steps required to activate/deactivate AVIC.
>>
>> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
>> ---
>>   arch/x86/kvm/svm.c | 57 +++++++++++++++++++++++++++++++++++++++++++---
>>   1 file changed, 54 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>> index f41f34f70dde..84116e689d5f 100644
>> --- a/arch/x86/kvm/svm.c
>> +++ b/arch/x86/kvm/svm.c
>> @@ -391,6 +391,8 @@ static u8 rsm_ins_bytes[] = "\x0f\xaa";
>>   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_vcpu *vcpu);
>>
>>   static int nested_svm_exit_handled(struct vcpu_svm *svm);
>>   static int nested_svm_intercept(struct vcpu_svm *svm);
>> @@ -2109,6 +2111,9 @@ static void avic_set_running(struct kvm_vcpu *vcpu, bool is_run)
>>   {
>>        struct vcpu_svm *svm = to_svm(vcpu);
>>
>> +     if (!kvm_vcpu_apicv_active(vcpu))
>> +             return;
>> +
>>        svm->avic_is_running = is_run;
>>        if (is_run)
>>                avic_vcpu_load(vcpu, vcpu->cpu);
>> @@ -2356,6 +2361,10 @@ static void svm_vcpu_blocking(struct kvm_vcpu *vcpu)
>>
>>   static void svm_vcpu_unblocking(struct kvm_vcpu *vcpu)
>>   {
>> +     if (kvm_check_request(KVM_REQ_APICV_ACTIVATE, vcpu))
>> +             kvm_vcpu_activate_apicv(vcpu);
>> +     if (kvm_check_request(KVM_REQ_APICV_DEACTIVATE, vcpu))
>> +             kvm_vcpu_deactivate_apicv(vcpu);
>>        avic_set_running(vcpu, true);
>>   }
>>
>> @@ -4505,6 +4514,15 @@ 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))
>> +             svm_request_activate_avic(&svm->vcpu);
>> +
> 
> Are we sure, we're not accidentally re-enabling AVIC, if it was disabled via
> kvm_hv_activate_synic()?

Actually, I missed this case. Now I have a solution that I'll be send out for review in V2.

>>        svm->vmcb->control.int_ctl &= ~V_IRQ_MASK;
>>        mark_dirty(svm->vmcb, VMCB_INTR);
>>        ++svm->vcpu.stat.irq_window_exits;
>> @@ -5206,6 +5224,34 @@ static void svm_hwapic_isr_update(struct kvm_vcpu *vcpu, int max_isr)
>>   {
>>   }
>>
>> +static bool is_avic_active(struct vcpu_svm *svm)
>> +{
>> +     return (svm_get_enable_apicv(&svm->vcpu) &&
>> +             svm->vmcb->control.int_ctl & AVIC_ENABLE_MASK);
>> +}
>> +
>> +static void svm_request_activate_avic(struct kvm_vcpu *vcpu)
>> +{
>> +     struct vcpu_svm *svm = to_svm(vcpu);
>> +
>> +     if (!lapic_in_kernel(vcpu) || is_avic_active(svm))
>> +             return;
>> +
>> +     avic_setup_access_page(vcpu, false);
>> +     kvm_make_apicv_activate_request(vcpu->kvm);
>> +}
>> +
>> +static void svm_request_deactivate_avic(struct kvm_vcpu *vcpu)
>> +{
>> +     struct vcpu_svm *svm = to_svm(vcpu);
>> +
>> +     if (!lapic_in_kernel(vcpu) || !is_avic_active(svm))
>> +             return;
>> +
>> +     avic_destroy_access_page(vcpu);
> 
> Something like avic_destroy_access_page() is not called, when AVIC is
> disabled via kvm_hv_activate_synic().
> 
> Is that an oversight in the other code path, is it not needed here,
> or am I missing something?

This is an oversight. I also have a fix for this in V2.

Thanks,
Suravee