linux-kernel - RE: [PATCH 2/3] kvm: x86: do not use KVM_REQ

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Message-ID: <E959C4978C3B6342920538CF579893F0198811F4@SHSMSX104.ccr.corp.intel.com>
Date:   Wed, 28 Sep 2016 12:06:09 +0000
From:   "Wu, Feng" <feng.wu@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "yang.zhang.wz@...il.com" <yang.zhang.wz@...il.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "Wu, Feng" <feng.wu@...el.com>
Subject: RE: [PATCH 2/3] kvm: x86: do not use KVM_REQ_EVENT for APICv
 interrupt injection



> -----Original Message-----
> From: Paolo Bonzini [mailto:pbonzini@...hat.com]
> Sent: Wednesday, September 28, 2016 7:50 PM
> To: Wu, Feng <feng.wu@...el.com>; Michael S. Tsirkin <mst@...hat.com>
> Cc: linux-kernel@...r.kernel.org; kvm@...r.kernel.org;
> yang.zhang.wz@...il.com; rkrcmar@...hat.com
> Subject: Re: [PATCH 2/3] kvm: x86: do not use KVM_REQ_EVENT for APICv
> interrupt injection
> 
> 
> 
> On 28/09/2016 13:40, Wu, Feng wrote:
> > IIUIC, the issue you describe above is that IPI for posted-interrupts may be
> > issued between
> >
> > vcpu->mode = IN_GUEST_MODE;
> >
> > and
> >
> > local_irq_disable();
> >
> > But if that really happens, we will call kvm_vcpu_kick() in
> > vmx_deliver_posted_interrupt(), hence the vcpu->mode will be changed
> > to EXITING_GUEST_MODE, then we will goto cancel_injection in
> > vcpu_enter_guest, so the posted-interrupt will be delivered to guest
> > in the next vmentry. Seems I cannot see the problem. Do I miss something?
> 
> No, if that happens kvm_trigger_posted_interrupt returns true, hence
> kvm_vcpu_kick is not called.  

Oops, I missed the "!" before the function call ...

> With the fix, the IPI is processed as soon
> as the guest enters non-root mode, and the interrupt is injected.

Exactly!

> 
> 
> The other issue occurs when the IPI is sent 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.  Then the
> guest is entered with PIR.ON, but the PI interrupt is not pending and
> hence the interrupt is never delivered to the guest.  

Why "never", at least, the interrupt should be delivered to the guest in the next
vm-entry, right? I mean vm-entry -> vm-exit -> _vm-entry_ (interrupts will be
delivered at this vm-entery).

Thanks,
Feng

> The fix for this
> is to move the RVI update after IN_GUEST_MODE.  Then the source CPU uses
> the posted interrupt IPI instead of kvm_cpu_kick, and everything works.
> 
> Paolo