Re: [PATCH RFC v2 5/7] x86/iommu: Make x86-iommu a singleton object

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Oct 30, 2024 at 09:01:03AM -0400, Peter Xu wrote:
> On Wed, Oct 30, 2024 at 10:33:23AM +0000, Daniel P. Berrangé wrote:
> > On Tue, Oct 29, 2024 at 05:16:05PM -0400, Peter Xu wrote:
> > > X86 IOMMUs cannot be created more than one on a system yet.  Make it a
> > > singleton so it guards the system from accidentally create yet another
> > > IOMMU object when one already presents.
> > > 
> > > Now if someone tries to create more than one, e.g., via:
> > > 
> > >   ./qemu -M q35 -device intel-iommu -device intel-iommu
> > > 
> > > The error will change from:
> > > 
> > >   qemu-system-x86_64: -device intel-iommu: QEMU does not support multiple vIOMMUs for x86 yet.
> > > 
> > > To:
> > > 
> > >   qemu-system-x86_64: -device intel-iommu: Class 'intel-iommu' only supports one instance
> > > 
> > > Unfortunately, yet we can't remove the singleton check in the machine
> > > hook (pc_machine_device_pre_plug_cb), because there can also be
> > > virtio-iommu involved, which doesn't share a common parent class yet.
> > > 
> > > But with this, it should be closer to reach that goal to check singleton by
> > > QOM one day.
> > 
> > Looking at the other iommu impls, I noticed that they all have something
> > in common, in that they call pci_setup_iommu from their realize()
> > function to register their set of callback functions.
> > 
> > This pci_setup_iommu can happily be called multiple times and just
> > over-writes previously registered callbacks. I wonder if this is a better
> > place to diagnose incorrect usage of multiple impls. If pci_setup_iommu
> > raised an error, it wouldn't matter that virtio-iommu doesn't share
> > a common parent with intel-iommu. This would also perhaps be better for
> > a future heterogeneous machine types, where it might be valid to have
> > multiple iommus concurrently. Checking at the resource setup/registration
> > point reflects where the physical constraint comes from.
> 
> There can still be side effects that vIOMMU code, at least so far, consider
> it the only object even during init/realize.  E.g. vtd_decide_config() has
> kvm_enable_x2apic() calls which we definitely don't want to be triggered
> during machine running.  The pci_setup_iommu() idea could work indeed but
> it might still need cleanups here and there all over the places.

The side effects surely don't matter, because when we hit the error
scenario, we'll propagate that up the stack until something calls
exit(), since this is a cold boot path, rather than hotplug ?

With regards,
Daniel
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