linux-kernel - Re: [PATCH v2] KVM: VMX: Dont' send posted IRQ if vCPU == this vCPU and vCPU is IN_GUEST

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Date:   Thu, 20 Jan 2022 18:30:48 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v2] KVM: VMX: Dont' send posted IRQ if vCPU == this vCPU
 and vCPU is IN_GUEST_MODE

On 1/19/22 08:11, Wanpeng Li wrote:
> kindly ping, :)

Will look at it next week for 5.18.

Paolo

> On Sat, 8 Jan 2022 at 10:43, Wanpeng Li <kernellwp@...il.com> wrote:
>>
>> From: Wanpeng Li <wanpengli@...cent.com>
>>
>> When delivering a virtual interrupt, don't actually send a posted interrupt
>> if the target vCPU is also the currently running vCPU and is IN_GUEST_MODE,
>> in which case the interrupt is being sent from a VM-Exit fastpath and the
>> core run loop in vcpu_enter_guest() will manually move the interrupt from
>> the PIR to vmcs.GUEST_RVI.  IRQs are disabled while IN_GUEST_MODE, thus
>> there's no possibility of the virtual interrupt being sent from anything
>> other than KVM, i.e. KVM won't suppress a wake event from an IRQ handler
>> (see commit fdba608f15e2, "KVM: VMX: Wake vCPU when delivering posted IRQ
>> even if vCPU == this vCPU").
>>
>> Eliding the posted interrupt restores the performance provided by the
>> combination of commits 379a3c8ee444 ("KVM: VMX: Optimize posted-interrupt
>> delivery for timer fastpath") and 26efe2fd92e5 ("KVM: VMX: Handle
>> preemption timer fastpath").
>>
>> Thanks Sean for better comments.
>>
>> Suggested-by: Chao Gao <chao.gao@...el.com>
>> Reviewed-by: Sean Christopherson <seanjc@...gle.com>
>> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
>> ---
>>   arch/x86/kvm/vmx/vmx.c | 40 +++++++++++++++++++++-------------------
>>   1 file changed, 21 insertions(+), 19 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index fe06b02994e6..e06377c9a4cf 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -3908,31 +3908,33 @@ static inline void kvm_vcpu_trigger_posted_interrupt(struct kvm_vcpu *vcpu,
>>   #ifdef CONFIG_SMP
>>          if (vcpu->mode == IN_GUEST_MODE) {
>>                  /*
>> -                * The vector of interrupt to be delivered to vcpu had
>> -                * been set in PIR before this function.
>> +                * The vector of the virtual has already been set in the PIR.
>> +                * Send a notification event to deliver the virtual interrupt
>> +                * unless the vCPU is the currently running vCPU, i.e. the
>> +                * event is being sent from a fastpath VM-Exit handler, in
>> +                * which case the PIR will be synced to the vIRR before
>> +                * re-entering the guest.
>>                   *
>> -                * Following cases will be reached in this block, and
>> -                * we always send a notification event in all cases as
>> -                * explained below.
>> +                * When the target is not the running vCPU, the following
>> +                * possibilities emerge:
>>                   *
>> -                * Case 1: vcpu keeps in non-root mode. Sending a
>> -                * notification event posts the interrupt to vcpu.
>> +                * Case 1: vCPU stays in non-root mode. Sending a notification
>> +                * event posts the interrupt to the vCPU.
>>                   *
>> -                * Case 2: vcpu exits to root mode and is still
>> -                * runnable. PIR will be synced to vIRR before the
>> -                * next vcpu entry. Sending a notification event in
>> -                * this case has no effect, as vcpu is not in root
>> -                * mode.
>> +                * Case 2: vCPU exits to root mode and is still runnable. The
>> +                * PIR will be synced to the vIRR before re-entering the guest.
>> +                * Sending a notification event is ok as the host IRQ handler
>> +                * will ignore the spurious event.
>>                   *
>> -                * Case 3: vcpu exits to root mode and is blocked.
>> -                * vcpu_block() has already synced PIR to vIRR and
>> -                * never blocks vcpu if vIRR is not cleared. Therefore,
>> -                * a blocked vcpu here does not wait for any requested
>> -                * interrupts in PIR, and sending a notification event
>> -                * which has no effect is safe here.
>> +                * Case 3: vCPU exits to root mode and is blocked. vcpu_block()
>> +                * has already synced PIR to vIRR and never blocks the vCPU if
>> +                * the vIRR is not empty. Therefore, a blocked vCPU here does
>> +                * not wait for any requested interrupts in PIR, and sending a
>> +                * notification event also results in a benign, spurious event.
>>                   */
>>
>> -               apic->send_IPI_mask(get_cpu_mask(vcpu->cpu), pi_vec);
>> +               if (vcpu != kvm_get_running_vcpu())
>> +                       apic->send_IPI_mask(get_cpu_mask(vcpu->cpu), pi_vec);
>>                  return;
>>          }
>>   #endif
>> --
>> 2.25.1
>>
>