On 14/04/21 04:28, Lai Jiangshan wrote:
On Tue, Apr 13, 2021 at 8:15 PM Paolo Bonzini <pbonzini@xxxxxxxxxx> 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
