On 24/07/2017 10:57, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>
> Preemption can occur in the preemption timer expiration handler:
>
> CPU0 CPU1
>
> preemption timer vmexit
> handle_preemption_timer(vCPU0)
> kvm_lapic_expired_hv_timer
> hv_timer_is_use == true
> sched_out
> sched_in
> kvm_arch_vcpu_load
> kvm_lapic_restart_hv_timer
> restart_apic_timer
> start_hv_timer
> already-expired timer or sw timer triggerd in the window
> start_sw_timer
> cancel_hv_timer
At this point, the timer interrupt is injected, right?
If this is correct, kvm_lapic_expired_hv_timer can just do nothing if
the timer is not in use, with a comment explaining that the preemption
notifier has run start_sw_timer and thus injected the timer interrupt.
> /* back in kvm_lapic_expired_hv_timer */
> cancel_hv_timer
> WARN_ON(!apic->lapic_timer.hv_timer_in_use); ==> Oops
>
> This can be reproduced if CONFIG_PREEMPT is enabled.
>
> This patch fixes it by don't cancel preemption timer repeatedly if
> the preemption timer has already been cancelled due to preemption
> since already-expired timer or sw timer triggered in the window.
>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
> arch/x86/kvm/lapic.c | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 2819d4c..8341b40 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1560,9 +1560,13 @@ void kvm_lapic_expired_hv_timer(struct kvm_vcpu *vcpu)
> {
> struct kvm_lapic *apic = vcpu->arch.apic;
>
> - WARN_ON(!apic->lapic_timer.hv_timer_in_use);
> - WARN_ON(swait_active(&vcpu->wq));
> - cancel_hv_timer(apic);
> + preempt_disable();
> + if (!(!apic_lvtt_period(apic) && atomic_read(&apic->lapic_timer.pending))) {
Why is the "if" necessary?
Maybe all of kvm_lapic_expired_hv_timer and start_sw_timer should be in
preemption-disabled regions, which trivially avoids any reentrancy issue
with the preempt notifier. Then, cancel_hv_timer can assert that it's
called with preemption disabled.
Paolo
> + WARN_ON(!apic->lapic_timer.hv_timer_in_use);
> + WARN_ON(swait_active(&vcpu->wq));
> + cancel_hv_timer(apic);
> + }
> + preempt_enable();
> apic_timer_expired(apic);
>
> if (apic_lvtt_period(apic) && apic->lapic_timer.period) {
>
