From: Paolo Bonzini <pbonzini@redhat.com>
To: Lai Jiangshan <jiangshanlai+lkml@gmail.com>
Cc: Sean Christopherson <seanjc@google.com>,
LKML <linux-kernel@vger.kernel.org>,
kvm@vger.kernel.org, Filippo Sironi <sironi@amazon.de>,
David Woodhouse <dwmw@amazon.co.uk>,
"v4.7+" <stable@vger.kernel.org>,
Wanpeng Li <wanpengli@tencent.com>
Subject: Re: [PATCH 2/2] KVM: x86: Fix split-irqchip vs interrupt injection window request
Date: Wed, 14 Apr 2021 18:58:49 +0200 [thread overview]
Message-ID: <6d9dafb1-b8ff-82ef-93dc-da869fe7ba0f@redhat.com> (raw)
In-Reply-To: <CAJhGHyChfXdcAMzzD7P3aC8tnhFW5GvOt88vOY=D3pyb7hgNAA@mail.gmail.com>
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
next prev parent reply other threads:[~2021-04-14 16:58 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-27 11:21 [PATCH 0/2] KVM: x86: cleanup and fix userspace interrupt window Paolo Bonzini
2020-11-27 11:21 ` [PATCH 1/2] KVM: x86: handle !lapic_in_kernel case in kvm_cpu_*_extint Paolo Bonzini
2020-11-27 11:56 ` David Woodhouse
2020-11-27 12:52 ` Filippo Sironi
2020-11-27 11:21 ` [PATCH 2/2] KVM: x86: Fix split-irqchip vs interrupt injection window request Paolo Bonzini
2020-11-27 12:53 ` Filippo Sironi
2021-04-09 7:14 ` Lai Jiangshan
2021-04-12 21:43 ` Sean Christopherson
2021-04-13 11:03 ` Lai Jiangshan
2021-04-13 12:15 ` Paolo Bonzini
2021-04-14 2:28 ` Lai Jiangshan
2021-04-14 16:58 ` Paolo Bonzini [this message]
2021-04-15 0:59 ` Lai Jiangshan
2021-04-15 6:07 ` Paolo Bonzini
2021-04-15 8:06 ` Lai Jiangshan
2021-04-13 12:10 ` Paolo Bonzini
2020-11-27 12:49 ` [PATCH 0/2] KVM: x86: cleanup and fix userspace interrupt window David Woodhouse
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