[PATCH] KVM: arm64: Don't eagerly teardown the vgic on init error

[Oliver Upton]

On Thu, Oct 10, 2024 at 01:47:05PM +0100, Marc Zyngier wrote:
> On Thu, 10 Oct 2024 09:47:04 +0100, Oliver Upton <oliver.upton at linux.dev> wrote:
> > A small but stupid window to race with.
> 
> Ah, gotcha. I guess getting rid of the early-out in
> kvm_vgic_map_resources() would plug that one. Want to post a fix for
> that?

Yep, will do.

> > 
> > > > If memory serves, kvm_vgic_map_resources() used to do all of this behind
> > > > the config_lock to cure the race, but that wound up inverting lock
> > > > ordering on srcu.
> > > 
> > > Probably something like that. We also used to hold the kvm lock, which
> > > made everything much simpler, but awfully wrong.
> > > 
> > > > Note to self: Impose strict ordering on GIC initialization v. vCPU
> > > > creation if/when we get a new flavor of irqchip.
> > > 
> > > One of the things we should have done when introducing GICv3 is to
> > > impose that at KVM_DEV_ARM_VGIC_CTRL_INIT, the GIC memory map is
> > > final. I remember some push-back on the QEMU side of things, as they
> > > like to decouple things, but this has proved to be a nightmare.
> > 
> > Pushing more of the initialization complexity into userspace feels like
> > the right thing. Since we clearly have no idea what we're doing :)
> 
> KVM APIv2?

Even better, we can just go straight to v3 and skip all the mistakes we
would've made in v2.

> > 
> > > > The crappy assumption here is kvm_arch_vcpu_run_pid_change() and its
> > > > callees are allowed to destroy VM-scoped structures in error handling.
> > > 
> > > I think this is symptomatic of more general issue: we perform VM-wide
> > > configuration in the context of a vcpu. We have tons of this stuff to
> > > paper over the lack of a "this VM is fully configured" barrier.
> > > 
> > > I wonder whether we could sidestep things by punting the finalisation
> > > of the VM to a different context (workqueue?)  and simply return
> > > -EAGAIN or -EINTR to userspace while we're processing it. That doesn't
> > > solve the "I'm missing parts of the address map and I'm going to die"
> > > part though.
> > 
> > Throwing it back at userspace would be nice, but unfortunately for ABI I
> > think we need to block/spin vCPUs in the kernel til the VM is in fully
> > working condition. A fragile userspace could explode for a 'spurious'
> > EAGAIN/EINTR where there wasn't one before.
> 
> EINTR needs to be handled already, as this is how you report
> preemption by a signal.

Of course, I'm just assuming userspace is mean and will complain if no
signal actually arrives.

-- 
Thanks,
Oliver