RE: The vcpu won't be wakened for a long time

["Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" <longpeng2@huawei.com>]

> -----Original Message-----
> From: Sean Christopherson [mailto:seanjc@google.com]
> Sent: Thursday, December 16, 2021 11:43 PM
> To: Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
> <longpeng2@huawei.com>
> Cc: pbonzini@redhat.com; kvm@vger.kernel.org; Gonglei (Arei)
> <arei.gonglei@huawei.com>; Huangzhichao <huangzhichao@huawei.com>; Wanpeng Li
> <wanpengli@tencent.com>; Vitaly Kuznetsov <vkuznets@redhat.com>; Jim Mattson
> <jmattson@google.com>; Joerg Roedel <joro@8bytes.org>; linux-kernel
> <linux-kernel@vger.kernel.org>
> Subject: Re: The vcpu won't be wakened for a long time
> 
> On Thu, Dec 16, 2021, Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
> wrote:
> > > What kernel version?  There have been a variety of fixes/changes in the
> > > area in recent kernels.
> >
> > The kernel version is 4.18, and it seems the latest kernel also has this problem.
> >
> > The following code can fixes this bug, I've tested it on 4.18.
> >
> > (4.18)
> >
> > @@ -3944,6 +3944,11 @@ static void vmx_deliver_posted_interrupt(struct
> kvm_vcpu *vcpu, int vector)
> >         if (pi_test_and_set_on(&vmx->pi_desc))
> >                 return;
> >
> > +       if (swq_has_sleeper(kvm_arch_vcpu_wq(vcpu))) {
> > +               kvm_vcpu_kick(vcpu);
> > +               return;
> > +       }
> > +
> >         if (vcpu != kvm_get_running_vcpu() &&
> >                 !kvm_vcpu_trigger_posted_interrupt(vcpu, false))
> >                 kvm_vcpu_kick(vcpu);
> >
> >
> > (latest)
> >
> > @@ -3959,6 +3959,11 @@ static int vmx_deliver_posted_interrupt(struct kvm_vcpu
> *vcpu, int vector)
> >         if (pi_test_and_set_on(&vmx->pi_desc))
> >                 return 0;
> >
> > +       if (rcuwait_active(&vcpu->wait)) {
> > +               kvm_vcpu_kick(vcpu);
> > +               return 0;
> > +       }
> > +
> >         if (vcpu != kvm_get_running_vcpu() &&
> >             !kvm_vcpu_trigger_posted_interrupt(vcpu, false))
> >                 kvm_vcpu_kick(vcpu);
> >
> > Do you have any suggestions ?
> 
> Hmm, that strongly suggests the "vcpu != kvm_get_running_vcpu()" is at fault.
> Can you try running with the below commit?  It's currently sitting in kvm/queue,
> but not marked for stable because I didn't think it was possible for the check
> to a cause a missed wake event in KVM's current code base.
> 

The below commit can fix the bug, we have just completed  the tests.
Thanks.

> commit 6a8110fea2c1b19711ac1ef718680dfd940363c6
> Author: Sean Christopherson <seanjc@google.com>
> Date:   Wed Dec 8 01:52:27 2021 +0000
> 
>     KVM: VMX: Wake vCPU when delivering posted IRQ even if vCPU == this vCPU
> 
>     Drop a check that guards triggering a posted interrupt on the currently
>     running vCPU, and more importantly guards waking the target vCPU if
>     triggering a posted interrupt fails because the vCPU isn't IN_GUEST_MODE.
>     The "do nothing" logic when "vcpu == running_vcpu" works only because KVM
>     doesn't have a path to ->deliver_posted_interrupt() from asynchronous
>     context, e.g. if apic_timer_expired() were changed to always go down the
>     posted interrupt path for APICv, or if the IN_GUEST_MODE check in
>     kvm_use_posted_timer_interrupt() were dropped, and the hrtimer fired in
>     kvm_vcpu_block() after the final kvm_vcpu_check_block() check, the vCPU
>     would be scheduled() out without being awakened, i.e. would "miss" the
>     timer interrupt.
> 
>     One could argue that invoking kvm_apic_local_deliver() from (soft) IRQ
>     context for the current running vCPU should be illegal, but nothing in
>     KVM actually enforces that rules.  There's also no strong obvious benefit
>     to making such behavior illegal, e.g. checking IN_GUEST_MODE and calling
>     kvm_vcpu_wake_up() is at worst marginally more costly than querying the
>     current running vCPU.
> 
>     Lastly, this aligns the non-nested and nested usage of triggering posted
>     interrupts, and will allow for additional cleanups.
> 
>     Signed-off-by: Sean Christopherson <seanjc@google.com>
>     Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
>     Message-Id: <20211208015236.1616697-18-seanjc@google.com>
>     Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 38749063da0e..f61a6348cffd 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -3995,8 +3995,7 @@ static int vmx_deliver_posted_interrupt(struct kvm_vcpu
> *vcpu, int vector)
>          * guaranteed to see PID.ON=1 and sync the PIR to IRR if triggering a
>          * posted interrupt "fails" because vcpu->mode != IN_GUEST_MODE.
>          */
> -       if (vcpu != kvm_get_running_vcpu() &&
> -           !kvm_vcpu_trigger_posted_interrupt(vcpu, false))
> +       if (!kvm_vcpu_trigger_posted_interrupt(vcpu, false))
>                 kvm_vcpu_wake_up(vcpu);
> 
>         return 0;