Re: [PATCH 6/6] kvm: x86: do not use KVM_REQ_EVENT for APICv interrupt injection

Wanpeng Li
2017-03-09T09:40:53Z

2017-03-09 9:23 GMT+08:00 Wanpeng Li <kernellwp@gmail.com>:
> 2016-12-20 0:17 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
>> Since bf9f6ac8d749 ("KVM: Update Posted-Interrupts Descriptor when vCPU
>> is blocked", 2015-09-18) the posted interrupt descriptor is checked
>> unconditionally for PIR.ON. Therefore we don't need KVM_REQ_EVENT to
>> trigger the scan and, if NMIs or SMIs are not involved, we can avoid
>> the complicated event injection path.
>>
>> Calling kvm_vcpu_kick if PIR.ON=1 is also useless, though it has been
>> there since APICv was introduced.
>>
>> However, without the KVM_REQ_EVENT safety net KVM needs to be much
>> more careful about races between vmx_deliver_posted_interrupt and
>> vcpu_enter_guest. First, the IPI for posted interrupts may be issued
>> between setting vcpu->mode = IN_GUEST_MODE and disabling interrupts.
>> If that happens, kvm_trigger_posted_interrupt returns true, but
>> smp_kvm_posted_intr_ipi doesn't do anything about it. The guest is
>> entered with PIR.ON, but the posted interrupt IPI has not been sent
>> and the interrupt is only delivered to the guest on the next vmentry
>> (if any). To fix this, disable interrupts before setting vcpu->mode.
>> This ensures that the IPI is delayed until the guest enters non-root mode;
>> it is then trapped by the processor causing the interrupt to be injected.
>>
>> Second, the IPI may be issued between
>>
>> kvm_x86_ops->hwapic_irr_update(vcpu,
>> kvm_lapic_find_highest_irr(vcpu));
>>
>> and vcpu->mode = IN_GUEST_MODE. In this case, kvm_vcpu_kick is called
>> but it (correctly) doesn't do anything because it sees vcpu->mode ==
>> OUTSIDE_GUEST_MODE. Again, the guest is entered with PIR.ON but no
>> posted interrupt IPI is pending; this time, the fix for this is to move
>> the RVI update after IN_GUEST_MODE.
>>
>> Both issues were previously masked by the liberal usage of KVM_REQ_EVENT.
>> In both race scenarios KVM_REQ_EVENT would cancel guest entry, resulting
>> in another vmentry which would inject the interrupt.
>>
>> This saves about 300 cycles on the self_ipi_* tests of vmexit.flat.
>>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>> arch/x86/kvm/lapic.c | 11 ++++-------
>> arch/x86/kvm/vmx.c | 8 +++++---
>> arch/x86/kvm/x86.c | 44 +++++++++++++++++++++++++-------------------
>> 3 files changed, 34 insertions(+), 29 deletions(-)
>>
>> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
>> index f644dd1dbe71..5ea94b622e88 100644
>> --- a/arch/x86/kvm/lapic.c
>> +++ b/arch/x86/kvm/lapic.c
>> @@ -385,12 +385,8 @@ int __kvm_apic_update_irr(u32 *pir, void *regs)
>> int kvm_apic_update_irr(struct kvm_vcpu *vcpu, u32 *pir)
>> {
>> struct kvm_lapic *apic = vcpu->arch.apic;
>> - int max_irr;
>>
>> - max_irr = __kvm_apic_update_irr(pir, apic->regs);
>> -
>> - kvm_make_request(KVM_REQ_EVENT, vcpu);
>> - return max_irr;
>> + return __kvm_apic_update_irr(pir, apic->regs);
>> }
>> EXPORT_SYMBOL_GPL(kvm_apic_update_irr);
>>
>> @@ -423,9 +419,10 @@ static inline void apic_clear_irr(int vec, struct kvm_lapic *apic)
>> vcpu = apic->vcpu;
>>
>> if (unlikely(vcpu->arch.apicv_active)) {
>> - /* try to update RVI */
>> + /* need to update RVI */
>> apic_clear_vector(vec, apic->regs + APIC_IRR);
>> - kvm_make_request(KVM_REQ_EVENT, vcpu);
>> + kvm_x86_ops->hwapic_irr_update(vcpu,
>> + apic_find_highest_irr(apic));
>> } else {
>> apic->irr_pending = false;
>> apic_clear_vector(vec, apic->regs + APIC_IRR);
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 27e40b180242..3dd4fad35a3e 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -5062,9 +5062,11 @@ static void vmx_deliver_posted_interrupt(struct kvm_vcpu *vcpu, int vector)
>> if (pi_test_and_set_pir(vector, &vmx->pi_desc))
>> return;
>>
>> - r = pi_test_and_set_on(&vmx->pi_desc);
>> - kvm_make_request(KVM_REQ_EVENT, vcpu);
>> - if (r || !kvm_vcpu_trigger_posted_interrupt(vcpu))
>> + /* If a previous notification has sent the IPI, nothing to do. */
>> + if (pi_test_and_set_on(&vmx->pi_desc))
>> + return;
>> +
>> + if (!kvm_vcpu_trigger_posted_interrupt(vcpu))
>> kvm_vcpu_kick(vcpu);
>> }
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index c666414adc1d..725473ba6dd3 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -6710,19 +6710,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>> kvm_hv_process_stimers(vcpu);
>> }
>>
>> - /*
>> - * KVM_REQ_EVENT is not set when posted interrupts are set by
>> - * VT-d hardware, so we have to update RVI unconditionally.
>> - */
>> - if (kvm_lapic_enabled(vcpu)) {
>> - /*
>> - * Update architecture specific hints for APIC
>> - * virtual interrupt delivery.
>> - */
>> - if (kvm_x86_ops->sync_pir_to_irr)
>> - kvm_x86_ops->sync_pir_to_irr(vcpu);
>> - }
>> -
>> if (kvm_check_request(KVM_REQ_EVENT, vcpu) || req_int_win) {
>> ++vcpu->stat.req_event;
>> kvm_apic_accept_events(vcpu);
>> @@ -6767,20 +6754,39 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>> kvm_x86_ops->prepare_guest_switch(vcpu);
>> if (vcpu->fpu_active)
>> kvm_load_guest_fpu(vcpu);
>> +
>> + /*
>> + * Disabling IRQs before setting IN_GUEST_MODE. Posted interrupt
>> + * IPI are then delayed after guest entry, which ensures that they
>> + * result in virtual interrupt delivery.
>> + */
>> + local_irq_disable();
>> vcpu->mode = IN_GUEST_MODE;
>>
>> srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
>>
>> /*
>> - * We should set ->mode before check ->requests,
>> - * Please see the comment in kvm_make_all_cpus_request.
>> - * This also orders the write to mode from any reads
>> - * to the page tables done while the VCPU is running.
>> - * Please see the comment in kvm_flush_remote_tlbs.
>> + * 1) We should set ->mode before checking ->requests. Please see
>> + * the comment in kvm_make_all_cpus_request.
>> + *
>> + * 2) For APICv, we should set ->mode before checking PIR.ON. This
>> + * pairs with the memory barrier implicit in pi_test_and_set_on
>> + * (see vmx_deliver_posted_interrupt).
>> + *
>> + * 3) This also orders the write to mode from any reads to the page
>> + * tables done while the VCPU is running. Please see the comment
>> + * in kvm_flush_remote_tlbs.
>> */
>> smp_mb__after_srcu_read_unlock();
>>
>> - local_irq_disable();
>
> The local_irq_disable() movement is unnecessary if you move sync_pir_to_irr.

In addition, this movement will increase the time of irq disable to
some degree. Do you think I can send a patch to revert it?

Regards,
Wanpeng Li

>
> - IPI after vcpu->mode = IN_GUEST_MODE and interrupt disable, PI is
> successfully.
> - IPI between vcpu->mode = IN_GUEST_MODE and interrupt disable, the
> sync_ir_to_irr will catch the PIR and set RVI.
>
> Regards,
> Wanpeng Li
>
>> + if (kvm_lapic_enabled(vcpu)) {
>> + /*
>> + * This handles the case where a posted interrupt was
>> + * notified with kvm_vcpu_kick.
>> + */
>> + if (kvm_x86_ops->sync_pir_to_irr)
>> + kvm_x86_ops->sync_pir_to_irr(vcpu);
>> + }
>>
>> if (vcpu->mode == EXITING_GUEST_MODE || vcpu->requests
>> || need_resched() || signal_pending(current)) {
>> --
>> 1.8.3.1
>>