Re: [Xen-devel] [PATCH v4] x86/apicv: fix RTC periodic timer and apicv issue

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Xuquan (Quan Xu) [mailto:xuquan8@xxxxxxxxxx]
> Sent: Wednesday, December 21, 2016 1:44 PM
> 
> 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.
> 
> And that we update periodic timer interrupt in every VM-entry, there is
> a chance that already-injected instance (before EOI-induced exit happens)
> will incur another pending IRR setting if there is a VM-exit happens
> between virtual interrupt injection (vIRR->0, vISR->1) and EOI-induced
> exit (vISR->0), since pt_intr_post hasn't been invoked yet, then the
> guest receives more periodic timer interrupt.
> 
> So we set eoi_exit_bitmap for intack.vector when it's higher than
> pending periodic time interrupts. This way we can guarantee there's
> always a chance to post periodic time interrupts when periodic time
> interrupts becomes the highest one.
> 
> Signed-off-by: Quan Xu <xuquan8@xxxxxxxxxx>

Thanks for your consistent work on fixing this tricky issue:

Acked-by: Kevin Tian <kevin.tian@xxxxxxxxx>

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