Re: [PATCH v5 0/6] KVM: arm64: nv: Implement nested stage-2 reverse map (new data structure)

[Wei-Lin Chang]

On Fri, Sep 04, 2026 at 08:49:14AM +0100, Marc Zyngier wrote:

[...]

> > > >
> > > > The L1 was started with 8 vCPUs and 32 GiB of RAM using:
> > > > 
> > > >   qemu-system-aarch64 -smp 8 -m 32G \
> > > >     -machine virt,accel=kvm,gic-version=3,virtualization=on \
> > > >     -cpu host -nographic -enable-kvm \
> > > >     -drive if=pflash,format=raw,readonly=on,file=pflash0_bak.img \
> > > >     -drive if=pflash,format=raw,file=pflash1_bak.img \
> > > >     -drive file=./ubuntu-vm.qcow2,format=qcow2,if=virtio,cache=none,aio=native \
> > > >     -nic user,model=virtio-net-pci,hostfwd=tcp::11234-:22 \
> > > >     -serial mon:stdio
> > > > 
> > > 
> > > Puzzling. If you are only running an L1 in VHE mode, there is no
> > > shadow S2, and therefore nothing to unmap. For shadow S2s to be built
> > > and affect the MMU notifiers, you need to run an L2.
> > 
> > I was thinking the same at first, but realized even with L1 in VHE mode
> > there is a small period of time where L1 runs in its EL1 during boot, so
> > one nested MMU will become valid for each vCPU. That causes
> > kvm_nested_s2_unmap() to iterate through the entire IPA space 8 times
> > (-smp 8).
> 
> It should be one nested MMU for the whole VM, not one per vcpu.
> That's assuming they share the same VMID+VTCR.

(sigh) Yes, rookie mistake from me as always.

> 
> > What I am curious about is whether one single notifier unmap is enough
> > to hang L1, or were there multiple notifier unmaps.
> > 
> > QEMU with -machine virt uses 40 IPA bits only, unmapping that takes:
> > 1024  (4KB pages,  unmapping 1GB per iteration)
> > 32768 (16KB pages, unmapping 32MB per iteration)
> > 2048  (64KB pages, unmapping 512MB per iteration)
> > iterations for each page size. There aren't many mappings in each
> > iteration too. Does this really take that long on real hardware (even if
> > this must be done 8 times)?
> 
> This should be close to being at zero cost, so something else is
> amiss.
> 
> Could you please have a look?

Of course.

Thanks,
Wei-Lin Chang

> 
> 	M.
> 
> -- 
> Jazz isn't dead. It just smells funny.