linux-kernel - Re: [PATCH 2/4] KVM: VMX: avoid double list add with VT-d posted interrupts

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Date:   Fri, 9 Jun 2017 15:41:44 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Longpeng <longpeng2@...wei.com>,
        Huangweidong <weidong.huang@...wei.com>,
        Gonglei <arei.gonglei@...wei.com>,
        wangxin <wangxinxin.wang@...wei.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH 2/4] KVM: VMX: avoid double list add with VT-d posted
 interrupts

On Fri, Jun 09, 2017 at 09:29:45AM +0200, Paolo Bonzini wrote:
> 
> 
> On 09/06/2017 04:50, Peter Xu wrote:
> > Even, I'm thinking whether we can unconditionally setup PDST only in
> > pi_load(), then post_block() only needs to handle the NV bit.
> 
> No, you can't do that without fiddling with the blocked_vcpu lists in
> pi_load.

Then how about we keep the blocked_vcpu list maintainance in
post_block(), but only let pi_load() handle the PDST?

(I really feel like they are two things - the blocked_vcpu list helps
 for the kick when wakeup happens; while PDST makes sure the PI is
 always pointing to the correct cpu)

> 
> > (PS. since I'm at here... could I ask why in pi_pre_block we need to
> >  udpate PDST as well? I guess that decides who will run the
> >  wakeup_handler code to kick the vcpu thread, but would that really
> >  matter?)
> 
> For this one it's a yes. :)  I think it's not needed anymore indeed
> after these patches; see this comment:
> 
>                 /*
>                  * The wakeup_handler expects the VCPU to be on the
>                  * blocked_vcpu_list that matches ndst.

Actually I was always unclear on what this sentense means: iiuc
blocked_vcpu_list is only a list of vcpus that may need a kick, so why
it has anything to do with PDST after all?

(or say, no matter what PDST is, we just kick the vcpu thread without
 doing anything else, do we?)

> Interrupts
>                  * are disabled so no preemption should happen, but
>                  * err on the side of safety.
>                  */
> 
> So we could add a WARN.

Thanks,

-- 
Peter Xu