Messages in this thread | | | Subject | Re: [PATCH 2/2] KVM: x86: Fix split-irqchip vs interrupt injection window request | From | Paolo Bonzini <> | Date | Wed, 14 Apr 2021 18:58:49 +0200 |
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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/a9441aea2780da8c93da1c73da860219f98438de/pkg/sentry/platform/kvm/bluepill_amd64_unsafe.go#L105 > > 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
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