Re: [Xen-devel] [PATCH v3] x86/apicv: fix RTC periodic timer and apicv issue

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

> From: Xuquan (Quan Xu) [mailto:xuquan8@xxxxxxxxxx]
> Sent: Friday, December 16, 2016 5:40 PM
> 
> From 89fffdd6b563b2723e24d17231715bb8c9f24f90 Mon Sep 17 00:00:00 2001
> From: Quan Xu <xuquan8@xxxxxxxxxx>
> Date: Fri, 16 Dec 2016 17:24:01 +0800
> Subject: [PATCH v3] 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.
> 
> 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>

I suppose you've verified this new version, but still would like get
your explicit confirmation - did you still see time accuracy issue
in your side? Have you tried other guest OS types other than 
Win7-32?

> ---
>  xen/arch/x86/hvm/vmx/intr.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/arch/x86/hvm/vmx/intr.c b/xen/arch/x86/hvm/vmx/intr.c
> index 639a705..d7a5716 100644
> --- a/xen/arch/x86/hvm/vmx/intr.c
> +++ b/xen/arch/x86/hvm/vmx/intr.c
> @@ -315,9 +315,17 @@ void vmx_intr_assist(void)
>          * Set eoi_exit_bitmap for periodic timer interrup to cause 
> EOI-induced VM
>          * exit, then pending periodic time interrups have the chance to be 
> injected
>          * for compensation
> +        * 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
>          */
> -        if (pt_vector != -1)
> -            vmx_set_eoi_exit_bitmap(v, pt_vector);
> +        if ( pt_vector != -1 ) {
> +            if ( intack.vector > pt_vector )
> +                vmx_set_eoi_exit_bitmap(v, intack.vector);
> +            else
> +                vmx_set_eoi_exit_bitmap(v, pt_vector);
> +        }

Above can be simplified as one line change:
        if ( pt_vector != -1 )
                vmx_set_eoi_exit_bitmap(v, intack.vector);

> 
>          /* we need update the RVI field */
>          __vmread(GUEST_INTR_STATUS, &status);
> @@ -334,7 +342,8 @@ void vmx_intr_assist(void)
>              __vmwrite(EOI_EXIT_BITMAP(i), 
> v->arch.hvm_vmx.eoi_exit_bitmap[i]);
>          }
> 
> -        pt_intr_post(v, intack);
> +        if ( intack.vector == pt_vector )
> +            pt_intr_post(v, intack);
>      }
>      else
>      {
> --
> 1.7.12.4

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