linux-kernel - Re: kvm-unit-test fail for split irqchip

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Message-ID: <20160913190145.GE15680@potion>
Date:   Tue, 13 Sep 2016 21:01:46 +0200
From:   Radim Krcmar <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     kvm <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: kvm-unit-test fail for split irqchip

2016-09-13 19:06+0800, Wanpeng Li:
> Add -kernel_irqchip=split
> ./x86-run x86/eventinj.flat
> 
> qemu-system-x86_64 -enable-kvm -machine kernel_irqchip=split -cpu host
> -device pc-testdev -device isa-debug-exit,iobase=0xf4,iosize=0x4 -vnc
> none -serial stdio -device pci-testdev -kernel x86/eventinj.flat
> enabling apic
> paging enabled
> cr0 = 80010011
> cr3 = 7fff000
> cr4 = 20
> Sending vec 33 and 62 and mask one with TPR
> irq1 running
> irq1 running
> After 33/62 TPR test
> FAIL: TPR
> irq0 running
> irq0 running
> 
> Both irq1 and irq0 are executing twice.
> 
>  qemu-system-x86-22794 [001] d..2 34591.708476: kvm_entry: vcpu 0
>  qemu-system-x86-22794 [001] ...1 34591.708478: kvm_exit: reason
> MSR_WRITE rip 0x401f33 info 0 0
>  qemu-system-x86-22794 [001] ...1 34591.708478: kvm_apic: apic_write
> APIC_EOI = 0x0
>  qemu-system-x86-22794 [001] ...1 34591.708479: kvm_eoi: apicid 0 vector 62
>  qemu-system-x86-22794 [001] ...1 34591.708479: kvm_msr: msr_write 80b = 0x0
>  qemu-system-x86-22794 [001] d..2 34591.708480: kvm_entry: vcpu 0
>  qemu-system-x86-22794 [001] ...1 34591.708482: kvm_exit: reason
> PENDING_INTERRUPT rip 0x401f35 info 0 0
>  qemu-system-x86-22794 [001] ...1 34591.708482: kvm_userspace_exit:
> reason KVM_EXIT_IRQ_WINDOW_OPEN (7)
>  qemu-system-x86-22794 [001] ...1 34591.708491: kvm_inj_virq: irq 62
>  qemu-system-x86-22794 [001] d..2 34591.708492: kvm_entry: vcpu 0
>  qemu-system-x86-22794 [001] ...1 34591.708493: kvm_exit: reason
> IO_INSTRUCTION rip 0x4016ec info 3fd0008 0
> 
> From the trace we can see there is an interrupt window exit after the
> first interrupt EOI(irq 62), and the same irq(62) is injected
> duplicately after the interrupt window.
> 
> The bug can disappear if kernel_irqchip is on or -x2apic, the virtual
> x2apic mode will not be set due to commit (8d14695f9542 x86, apicv:
> add virtual x2apic support), so that tpr shadow in the x2apic doesn't
> work and wrmsr TPR register will trigger vmexit, and then kvmvapic
> will be used to optimize flexpriority=N or AMD. The
> report_trp_access() which is called in kvm_lapic_reg_write() will
> trigger a userspace exit.
> 
> TPR report access callbacks in qemu, kvm_handle_tpr_access() ->
> apic_handle_tpr_access_report() -> vapic_report_tpr_access() ->
> cpu_synchronize_state() will get apic register states from kvm.
> 
> Later, kvm_arch_pre_run -> cpu_get_pic_interrupt(if there is a pic
> interrupt) -> apic_get_interrupt,  it is a pic interrupt, however it
> gets the stale irq from apic register sync by report tpr access and
> KVM_INTERRUPT the second duplicate interrupt.
> 
> Paolo pointed out it is not the TPR associated bug, and we should
> figure out why there is an interrupt window exit after the first EOI.

Yeah, seems like QEMU bug.  QEMU does KVM_INTERRUPT(62) ioctl after KVM
exits with KVM_EXIT_IRQ_WINDOW_OPEN, which QEMU requested while the
guest was printing.  The printing calls

  serial_update_irq() -> qemu_irq_lower() -> qemu_set_irq() ->
  gsi_handler() -> qemu_set_irq() -> pic_irq_request() ->
  apic_deliver_pic_intr() -> kvm_handle_interrupt()

kvm_handle_interrupt() does

  interrupt_request |= CPU_INTERRUPT_HARD

which later calls cpu_get_pic_interrupt() in kvm_arch_pre_run(), but
that function uses stale information from APIC and injects 62 again.
If we synchronized the APIC, then the test would #GP, because there
would be no injectable interrupt in LAPIC or PIC, so pic_read_irq()
would return 15, thinking it was spurious.

I think the bug starts in pic_irq_request(), which should not touch
LAPIC.  The patch below makes it work (just the second hunk is
sufficient), but it's still far from sane code ...


diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 47593b741a5b..6983e9f13813 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -161,13 +161,16 @@ int cpu_get_pic_interrupt(CPUX86State *env)
     X86CPU *cpu = x86_env_get_cpu(env);
     int intno;
 
-    intno = apic_get_interrupt(cpu->apic_state);
-    if (intno >= 0) {
-        return intno;
-    }
-    /* read the irq from the PIC */
-    if (!apic_accept_pic_intr(cpu->apic_state)) {
-        return -1;
+    if (!kvm_irqchip_is_split()) {
+        /* XXX: why query APIC at all? */
+        intno = apic_get_interrupt(cpu->apic_state);
+        if (intno >= 0) {
+            return intno;
+        }
+        /* read the irq from the PIC */
+        if (!apic_accept_pic_intr(cpu->apic_state)) {
+            return -1;
+        }
     }
 
     intno = pic_read_irq(isa_pic);
@@ -180,7 +183,7 @@ static void pic_irq_request(void *opaque, int irq, int level)
     X86CPU *cpu = X86_CPU(cs);
 
     DPRINTF("pic_irqs: %s irq %d\n", level? "raise" : "lower", irq);
-    if (cpu->apic_state) {
+    if (cpu->apic_state && !kvm_irqchip_is_split()) {
         CPU_FOREACH(cs) {
             cpu = X86_CPU(cs);
             if (apic_accept_pic_intr(cpu->apic_state)) {