Re: [Xen-devel] [RFC PATCH] x86/apicv: fix RTC periodic timer and apicv issue

[Yang Zhang <yang.zhang.wz@xxxxxxxxx>]

2016年8月22日星期一，Xuquan (Euler) <xuquan8@xxxxxxxxxx> 写道：
> On August 22, 2016 8:04 PM, <JBeulich@xxxxxxxx> wrote:
> >>>> On 22.08.16 at 13:41, <xuquan8@xxxxxxxxxx> wrote:
> >> On August 22, 2016 6:36 PM, <JBeulich@xxxxxxxx> wrote:
> >>>>>> On 19.08.16 at 14:58, <xuquan8@xxxxxxxxxx> wrote:
> >>>> From 9b2df963c13ad27e2cffbeddfa3267782ac3da2a Mon Sep 17 00:00:00
> >>>> 2001
> >>>> From: Quan Xu <xuquan8@xxxxxxxxxx>
> >>>> Date: Fri, 19 Aug 2016 20:40:31 +0800
> >>>> Subject: [RFC PATCH] x86/apicv: fix RTC periodic timer and apicv
> >>>> issue
> >>>>
> >>>> When Xen apicv is enabled, wall clock time is faster on Windows7-32
> >>>> guest with high payload (with 2vCPU, captured from xentrace, in high
> >>>> payload, the count of IPI interrupt increases rapidly between these vCPUs).
> >>>>
> >>>> If IPI intrrupt (vector 0xe1) and periodic timer interrupt (vector
> >>>> 0xd1) are both pending (index of bit set in VIRR), unfortunately,
> >>>> the IPI intrrupt is high priority than periodic timer interrupt. Xen
> >>>> updates IPI interrupt bit set in VIRR to guest interrupt status
> >>>> (RVI) as a high priority and apicv (Virtual-Interrupt Delivery)
> >>>> delivers IPI interrupt within VMX non-root operation without a VM
> >>>> exit. Within VMX non-root operation, if periodic timer interrupt
> >>>> index of bit is set in VIRR and highest, the apicv delivers periodic timer
> >interrupt within VMX non-root operation as well.
> >>>>
> >>>> But in current code, if Xen doesn't update periodic timer interrupt
> >>>> bit set in VIRR to guest interrupt status (RVI) directly, Xen is not
> >>>> aware of this case to decrease the count (pending_intr_nr) of
> >>>> pending periodic timer interrupt, then Xen will deliver a periodic
> >>>> timer interrupt again. The guest receives more periodic timer
> >>>> interrupt.
> >>>>
> >>>> If the periodic timer interrut is delivered and not the highest
> >>>> priority, make Xen be aware of this case to decrease the count of
> >>>> pending periodic timer interrupt.
> >>>
> >>>I can see the issue you're trying to address, but for one - doesn't
> >>>this lead to other double accounting, namely once the pt irq becomes
> >>>the highest priority one?
> >>>
> >>
> >> It is does NOT lead to other double accounting..
> >> As if the pt irq becomes the highest priority one, the intack is pt one..
> >> Then:
> >>
> >>  +        else
> >>  +            pt_intr_post(v, intack);
> >
> >As just said in reply to Yang: If this is still the same interrupt instance as in a
> >prior run through here which took the if() branch, this change looks like having
> >the potential of double accounting.
> >
>
> I very appreciate your detail review. It looks like, but actually it doesn't happen.
>
>  As the key parameter 'pt->irq_issued'..
>
> In pt_update_irq(), once the PT irq is issued, set the pt->irq_issued..
> In pt_intr_post(), clear the pt->irq_issued before touching the count 'pt->pending_intr_nr'..
>
> According to your assumption, at the second call to pt_intr_post(), As if 'pt->irq_issued' is clear, pt is NULL in is_pt_irq() check,
> then return, there is no chance to touch the count 'pt->pending_intr_nr'..
> ------
> void pt_intr_post(struct vcpu *v, struct hvm_intack intack)
> {
> ...
>     pt = is_pt_irq(v, intack);
>     if ( pt == NULL )
>     {
>         spin_unlock(&v->arch.hvm_vcpu.tm_lock);
>         return;
>     }
> ...
>
>
> ...
> }
>
>
> static struct periodic_time *is_pt_irq()
> {
>  ....
>     list_for_each_entry ( pt, head, list )
>     {
>         if ( pt->pending_intr_nr && ________pt->irq_issued_______ &&
>              (intack.vector == pt_irq_vector(pt, intack.source)) )
>             return pt;
>     }
>
>     return NULL;
> }
>
>
>
>
>
> __IIUC__, this question is based on the following pseudocode detail the behavior of virtual-interrupt delivery is __not__ atomic:
>
> Vector <- RVI;
> VISR[Vector] <- 1;
> SVI <- Vector;
> VPPR<- Vector & F0H;
> VIRR[Vector] <- 0;
> IF any bits set in VIRR
>    Then RVI<-highest index of bit set in VIRR
>    ELSE RVI <-0
> FI
> Deliver interrupt with Vector through IDT
> ...
>
>
> We'd better check this first, as Yang said, this is atomic..

i have said that this is ensured by hardware.

 

> Quan


--

best regards

yang

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