Re: [PATCH 2/3] kvm: x86: do not use KVM_REQ_EVENT for APICv interrupt injection

[Paolo Bonzini]

On 28/09/2016 12:04, Wu, Feng wrote:
> 
> 
>> -----Original Message-----
>> From: Paolo Bonzini [mailto:paolo.bonzini@xxxxxxxxx] On Behalf Of Paolo
>> Bonzini
>> Sent: Wednesday, September 28, 2016 5:20 AM
>> To: linux-kernel@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx
>> Cc: yang.zhang.wz@xxxxxxxxx; Wu, Feng <feng.wu@xxxxxxxxx>;
>> mst@xxxxxxxxxx; rkrcmar@xxxxxxxxxx
>> Subject: [PATCH 2/3] kvm: x86: do not use KVM_REQ_EVENT for APICv interrupt
>> injection
>>
>> 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.
> 
> But the following code still remains in the KVM_REQ_EVENT checking part:
> 
> if (kvm_lapic_enabled(vcpu)) {
> 	update_cr8_intercept(vcpu);
> 	kvm_lapic_sync_to_vapic(vcpu);
> }
> 
> Does this matter?

Good question, but it doesn't matter for APICv because:

- update_cr8_intercept is disabled if APICv, see vmx.c:

        if (enable_apicv)
                kvm_x86_ops->update_cr8_intercept = NULL;

- kvm_lapic_sync_to_vapic's call to apic_sync_pv_eoi_to_guest is also
disabled if APICv:

        if (!pv_eoi_enabled(vcpu) ||
            apic->irr_pending ||
            apic->highest_isr_cache == -1 ||
            kvm_ioapic_handles_vector(apic, apic->highest_isr_cache))
                return;

(highest_isr_cache is always -1 for APICv)

- The TPR/ISR/IRR shadow that kvm_lapic_sync_to_vapic writes is only
read by the  paravirtualized TPR access code in the vAPIC ROM
(pc-bios/optionrom/kvmvapic.S in the QEMU tree). That code never runs if
you don't get TPR access vmexits, and indeed TPR access vmexits never
happen if KVM uses APICv (or even only the old-style TPR shadowing).

Paolo

> Thanks,
> Feng
> 
>>
>> However, there is a race between vmx_deliver_posted_interrupt and
>> vcpu_enter_guest.  Fix it by disabling interrupts before vcpu->mode is
>> set to IN_GUEST_MODE.
>>
>> Calling kvm_vcpu_kick if PIR.ON=1 is also useless, though it has been
>> there since APICv was introduced.
>>
>> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> ---
>>  arch/x86/kvm/lapic.c | 2 --
>>  arch/x86/kvm/vmx.c   | 8 +++++---
>>  arch/x86/kvm/x86.c   | 9 +++++++--
>>  3 files changed, 12 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
>> index 63a442aefc12..be8b7ad56dd1 100644
>> --- a/arch/x86/kvm/lapic.c
>> +++ b/arch/x86/kvm/lapic.c
>> @@ -356,8 +356,6 @@ void kvm_apic_update_irr(struct kvm_vcpu *vcpu, u32
>> *pir)
>>  	struct kvm_lapic *apic = vcpu->arch.apic;
>>
>>  	__kvm_apic_update_irr(pir, apic->regs);
>> -
>> -	kvm_make_request(KVM_REQ_EVENT, vcpu);
>>  }
>>  EXPORT_SYMBOL_GPL(kvm_apic_update_irr);
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index b33eee395b00..207b9aa32915 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -4844,9 +4844,11 @@ 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 3ee8a91a78c3..604cfbfc5bee 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -6658,6 +6658,13 @@ 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);
>> +
>> +	/*
>> +	 * Disable IRQs before setting IN_GUEST_MODE, so that
>> +	 * posted interrupts with vcpu->mode == IN_GUEST_MODE
>> +	 * always result in virtual interrupt delivery.
>> +	 */
>> +	local_irq_disable();
>>  	vcpu->mode = IN_GUEST_MODE;
>>
>>  	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
>> @@ -6671,8 +6678,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>  	 */
>>  	smp_mb__after_srcu_read_unlock();
>>
>> -	local_irq_disable();
>> -
>>  	if (vcpu->mode == EXITING_GUEST_MODE || vcpu->requests
>>  	    || need_resched() || signal_pending(current)) {
>>  		vcpu->mode = OUTSIDE_GUEST_MODE;
>> --
>> 1.8.3.1
>>
>