On 14/04/21 04:28, Lai Jiangshan wrote:
On Tue, Apr 13, 2021 at 8:15 PM Paolo Bonzini pbonzini@redhat.com wrote:
On 13/04/21 13:03, Lai Jiangshan wrote:
This patch claims that it has a place to stash the IRQ when EFLAGS.IF=0, but inject_pending_event() seams to ignore EFLAGS.IF and queues the IRQ to the guest directly in the first branch of using "kvm_x86_ops.set_irq(vcpu)".
This is only true for pure-userspace irqchip. For split-irqchip, in which case the "place to stash" the interrupt is vcpu->arch.pending_external_vector.
For pure-userspace irqchip, KVM_INTERRUPT only cares about being able to stash the interrupt in vcpu->arch.interrupt.injected. It is indeed wrong for userspace to call KVM_INTERRUPT if the vCPU is not ready for interrupt injection, but KVM_INTERRUPT does not return an error.
Thanks for the reply.
May I ask what is the correct/practical way of using KVM_INTERRUPT ABI for pure-userspace irqchip.
gVisor is indeed a pure-userspace irqchip, it will call KVM_INTERRUPT when kvm_run->ready_for_interrupt_injection=1 (along with other conditions unrelated to our discussion).
https://github.com/google/gvisor/blob/a9441aea2780da8c93da1c73da860219f98438...
if kvm_run->ready_for_interrupt_injection=1 when expection pending or EFLAGS.IF=0, it would be unexpected for gVisor.
Not with EFLAGS.IF=0. For pending exception, there is code to handle it in inject_pending_event:
... if (!vcpu->arch.exception.pending) { if (vcpu->arch.nmi_injected) { static_call(kvm_x86_set_nmi)(vcpu); can_inject = false; } else if (vcpu->arch.interrupt.injected) { static_call(kvm_x86_set_irq)(vcpu); can_inject = false; } } ... if (vcpu->arch.exception.pending) { ... can_inject = false; } // this is vcpu->arch.interrupt.injected for userspace LAPIC if (kvm_cpu_has_injectable_intr(vcpu)) { r = can_inject ? static_call(kvm_x86_interrupt_allowed)(vcpu, true) : -EBUSY; if (r < 0) goto busy; ... }
so what happens is:
- the interrupt will not be injected before the exception
- KVM will schedule an immediate vmexit to inject the interrupt as well
- if (as is likely) the exception has turned off interrupts, the next call to inject_pending_event will reach static_call(kvm_x86_enable_irq_window) and the interrupt will only be injected when IF becomes 1 again.
Paolo