linux-kernel - Re: [RFC V2] LoongArch: KVM: Handle interrupt early before enabling irq

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Message-ID: <Z-QYwWxhBH_nvmWH@google.com>
Date: Wed, 26 Mar 2025 08:09:53 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: bibo mao <maobibo@...ngson.cn>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Huacai Chen <chenhuacai@...nel.org>, 
	Tianrui Zhao <zhaotianrui@...ngson.cn>, loongarch@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC V2] LoongArch: KVM: Handle interrupt early before enabling irq

On Tue, Mar 25, 2025, bibo mao wrote:
> Hi Paolo, Sean
> 
> This idea comes from x86, do you have any guidance or suggestion about it?
> 
> Also I notice that there is such irq_enable()/irq_disable() pair on x86, I
> do not know why it is so.

Because on AMD (SVM), IRQ VM-Exits don't consume the IRQ, i.e. the exit is purely
a notification.  KVM still needs to enable IRQs to actually handle the pending IRQ.
And if the IRQ that triggered VM-Exit is for the host's tick, then it's desirable
to handle the tick IRQ before guest_timing_exit_irqoff() so that the timeslice is
accounted to the guest, not the host (the tick IRQ arrived while the guest was
active).

On Intel (VMX), KVM always runs in a mode where the VM-Exit acknowledge/consumes
the IRQ, and so KVM _must_ manually call into the appropriate interrupt handler.

>     local_irq_enable();
>     ++vcpu->stat.exits;
>     local_irq_disable();
>     guest_timing_exit_irqoff();
>     local_irq_enable();
> 
> Regards
> Bibo Mao
> 
> On 2025/3/11 下午3:47, Bibo Mao wrote:
> > If interrupt arrive when vCPU is running, vCPU will exit because of
> > interrupt exception. Currently interrupt exception is handled after
> > local_irq_enable() is called, and it is handled by host kernel rather
> > than KVM hypervisor. It will introduce extra another interrupt
> > exception and then host will handle irq.
> > 
> > If KVM hypervisor detect that it is interrupt exception, interrupt
> > can be handle early in KVM hypervisor before local_irq_enable() is
> > called.

The correctness of this depends on how LoongArch virtualization processes IRQs.
If the IRQ is consumed by the VM-Exit, then manually handling the IRQ early is
both optimal and necessary for correctness.  If the IRQ is NOT consumed by the
VM-Exit, then manually calling the interrupt handler from KVM will result in every
IRQ effectively happening twice: once on the manual call, and against when KVM
enables IRQs and the "real" IRQ fires.