Re: [RFC V2] LoongArch: KVM: Handle interrupt early before enabling irq
From: Sean Christopherson
Date: Wed Mar 26 2025 - 11:17:14 EST
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.