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

From: Lai Jiangshan <jiangshanlai+lkml@gmail.com>
To: Paolo Bonzini <pbonzini@redhat.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 10:28:55 +0800	[thread overview]
Message-ID: <CAJhGHyChfXdcAMzzD7P3aC8tnhFW5GvOt88vOY=D3pyb7hgNAA@mail.gmail.com> (raw)
In-Reply-To: <80b013dc-0078-76f4-1299-3cff261ef7d8@redhat.com>

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.

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
>