[PATCH v3] KVM: nVMX: Fix attempting to emulate "Acknowledge interrupt on exit" when there is no interrupt which L1 requires to inject to L2

From: Wanpeng Li
Date: Wed Aug 02 2017 - 06:48:34 EST

From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>

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WARNING: CPU: 5 PID: 2288 at arch/x86/kvm/vmx.c:11124 nested_vmx_vmexit+0xd64/0xd70 [kvm_intel]
CPU: 5 PID: 2288 Comm: qemu-system-x86 Not tainted 4.13.0-rc2+ #7
RIP: 0010:nested_vmx_vmexit+0xd64/0xd70 [kvm_intel]
Call Trace:
vmx_check_nested_events+0x131/0x1f0 [kvm_intel]
? vmx_check_nested_events+0x131/0x1f0 [kvm_intel]
kvm_arch_vcpu_ioctl_run+0x5dd/0x1be0 [kvm]
? vmx_vcpu_load+0x1be/0x220 [kvm_intel]
? kvm_arch_vcpu_load+0x62/0x230 [kvm]
kvm_vcpu_ioctl+0x340/0x700 [kvm]
? kvm_vcpu_ioctl+0x340/0x700 [kvm]
? __fget+0xfc/0x210
? __fget+0x11d/0x210
? trace_hardirqs_on_thunk+0x1a/0x1c

This can be reproduced by booting L1 guest w/ 'noapic' grub parameter, which
means that tells the kernel to not make use of any IOAPICs that may be present
in the system.

Actually external_intr variable in nested_vmx_vmexit() is the req_int_win
variable passed from vcpu_enter_guest() which means that the L0's userspace
requests an irq window. I observed the scenario (!kvm_cpu_has_interrupt(vcpu) &&
L0's userspace reqeusts an irq window) is true, so there is no interrupt which
L1 requires to inject to L2, we should not attempt to emualte "Acknowledge
interrupt on exit" for the irq window requirement in this scenario.

SDM says that with acknowledge interrupt on exit, bit 31 of the VM-exit
interrupt information (valid interrupt) is always set to 1 on
EXIT_REASON_EXTERNAL_INTERRUPT. We don't want to break hypervisors
expecting an interrupt in that case, so we should do a userspace VM exit
when the window is open and then inject the userspace interrupt with a
VM exit.

Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
v2 -> v3:
* request an irq window and don't nested vmexit
v1 -> v2:
* update patch description
* check nested_exit_intr_ack_set() first

arch/x86/kvm/vmx.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 80b20e8..9ef2ec3 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -10761,7 +10761,8 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu, bool external_intr)
return 0;

- if ((kvm_cpu_has_interrupt(vcpu) || external_intr) &&
+ if ((kvm_cpu_has_interrupt(vcpu) ||
+ (external_intr && !nested_exit_intr_ack_set(vcpu))) &&
nested_exit_on_intr(vcpu)) {
if (vmx->nested.nested_run_pending)
return -EBUSY;