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@xxxxxxxxx> 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
