Re: [PATCH] KVM: irqchip: synchronize srcu only if needed

[Yi Wang]

Many thanks for your such kind and detailed reply, Sean!

On Sat, Jan 13, 2024 at 12:28 AM Sean Christopherson <seanjc@googlecom> wrote:
>
> +other KVM maintainers
>
> On Fri, Jan 12, 2024, Yi Wang wrote:
> > From: Yi Wang <foxywang@xxxxxxxxxxx>
> >
> > We found that it may cost more than 20 milliseconds very accidentally
> > to enable cap of KVM_CAP_SPLIT_IRQCHIP on a host which has many vms
> > already.
> >
> > The reason is that when vmm(qemu/CloudHypervisor) invokes
> > KVM_CAP_SPLIT_IRQCHIP kvm will call synchronize_srcu_expedited() and
> > might_sleep and kworker of srcu may cost some delay during this period.
>
> might_sleep() yielding is not justification for changing KVM.  That's more or
> less saying "my task got preempted and took longer to run".  Well, yeah.

Agree. But I suppose it may be one of the reasons that makes  time of
KVM_CAP_SPLIT_IRQCHIP delayed, of course, the kworker has the biggest
suspicion :)

>
> > Since this happens during creating vm, it's no need to synchronize srcu
> > now 'cause everything is not ready(vcpu/irqfd) and none uses irq_srcu now.

...

> And on x86, I'm pretty sure as of commit 654f1f13ea56 ("kvm: Check irqchip mode
> before assign irqfd"), which added kvm_arch_irqfd_allowed(), it's impossible for
> kvm_irq_map_gsi() to encounter a NULL irq_routing _on x86_.
>
> But I strongly suspect other architectures can reach kvm_irq_map_gsi() with a
> NULL irq_routing, e.g. RISC-V dynamically configures its interrupt controller,
> yet doesn't implement kvm_arch_intc_initialized().
>
> So instead of special casing x86, what if we instead have KVM setup an empty
> IRQ routing table during kvm_create_vm(), and then avoid this mess entirely?
> That way x86 and s390 no longer need to set empty/dummy routing when creating
> an IRQCHIP, and the worst case scenario of userspace misusing an ioctl() is no
> longer a NULL pointer deref.

To setup an empty IRQ routing table during kvm_create_vm() sounds a good idea,
at this time vCPU have not been created and kvm->lock is held so skipping
synchronization is safe here.

However, there is one drawback, if vmm wants to emulate irqchip
itself, e.g. qemu
with command line '-machine kernel-irqchip=off' may not need irqchip
in kernel. How
do we handle this issue?


---
Best wishes
Yi Wang