linux-kernel - Re: [PATCH 2/2] KVM: x86: Fix split-irqchip vs interrupt injection window request

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Message-ID: <CAJhGHyChfXdcAMzzD7P3aC8tnhFW5GvOt88vOY=D3pyb7hgNAA@mail.gmail.com>
Date:   Wed, 14 Apr 2021 10:28:55 +0800
From:   Lai Jiangshan <jiangshanlai+lkml@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        Filippo Sironi <sironi@...zon.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        "v4.7+" <stable@...r.kernel.org>,
        Wanpeng Li <wanpengli@...cent.com>
Subject: Re: [PATCH 2/2] KVM: x86: Fix split-irqchip vs interrupt injection
 window request

On Tue, Apr 13, 2021 at 8:15 PM Paolo Bonzini <pbonzini@...hat.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.

Thanks
Lai

>
> Ignoring the fact that this would be incorrect use of the API, are you
> saying that the incorrect injection was not possible before this patch?
>
> Paolo
>