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

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

Thanks a lot!

> From: Gao, Chao
> Sent: Thursday, December 22, 2016 4:02 AM
> 
> Hi, xuquan.
> I have tested it on my skylake server. W/o this patch the inaccurate
> wall clock time issue only exists in Win7-32 guest. Win7-64, Win8-32, Win8-64,
> Win10-32 ,Win10-64 and linux-4.8.0+ guests don't have this issue.
> W/ this v4 patch, the issue disappears in Win7-32 guest and no wall lock time
> related regression is found on Win7-64, Win8-32, Win8-64, Win10-32, Win10-64
> and linux-4.8.0+ guest.
> 
> In windows guest, the test procedure is
> 1. Create a windows guest with 2 vCPU
> 2. run the following .bat in guest
>     :abcd
>     echo 111111
>     goto abcd
> 
> 3. Start a stop-watch outside the guest and monitor the clock at the lower 
> right
>    corner in guest. After 120 seconds according the guest clock, stop the 
> stop-watch.
>    If the time shows in the stop-watch is about 120 seconds, then I think
>    there is no the above issue in the guest. Otherwise, the time is 
> inaccurate.
> 
> In Win7-32 case, the stop-watch time is about 70 seconds, so the clock in 
> guest is
> obviously inaccurate.
> 
> In linux guest, the test procedure is
> 1. Create a linux guest with 4 vCPU
> 2. insmod the following linux module
>    (through output of /proc/interrupt, about 850000 ipis in 13 seconds)
> 3. use date command to get guest time, others are same as test in windows 
> guest
> 
> #include <linux/init.h>
> #include <linux/module.h>
> #include <linux/kthread.h>
> #include <linux/sched.h>
> #include <asm/delay.h>
> MODULE_LICENSE("GPL");
> void workload(void *info)
> {
>         asm volatile("nop");
> }
> 
> void msleep(unsigned int msecs);
> static int ipi_generator(void * info)
> {
>         int i;
>         while (!kthread_should_stop()) {
>                 for(i=0; i< 5 * 10000; i++)
>                 {
>                         smp_call_function(workload, NULL,1);
>                 }
>                 msleep(1);
>         }
>         return 0;
> }
> struct task_struct *thread;
> static int __init ipi_init(void)
> {
>         thread = kthread_run(ipi_generator, NULL, "IPI");
>         if (IS_ERR(thread))
>                 return PTR_ERR(thread);
>         return 0;
> }
> 
> static void __exit ipi_exit(void)
> {
>         kthread_stop(thread);
> }
> module_init(ipi_init);
> module_exit(ipi_exit);
> 
> Are these tests sufficient? Please let me know if you have any other thoughts.
> 
> On Wed, Dec 21, 2016 at 05:44:08AM +0000, Xuquan (Quan Xu) wrote:
> >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>
> >---
> > xen/arch/x86/hvm/vmx/intr.c | 9 +++++++--
> > 1 file changed, 7 insertions(+), 2 deletions(-)
> >
> >diff --git a/xen/arch/x86/hvm/vmx/intr.c b/xen/arch/x86/hvm/vmx/intr.c
> >index 639a705..0cf26b4 100644
> >--- a/xen/arch/x86/hvm/vmx/intr.c
> >+++ b/xen/arch/x86/hvm/vmx/intr.c
> >@@ -315,9 +315,13 @@ 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);
> >+            vmx_set_eoi_exit_bitmap(v, intack.vector);
> >
> >         /* we need update the RVI field */
> >         __vmread(GUEST_INTR_STATUS, &status);
> >@@ -334,7 +338,8 @@ 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
> >     {
> >--
> >1.8.3.4

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