Re: [PATCH v7 12/17] vfio-user: IOMMU support for remote device

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Mar 29, 2022 at 07:58:51PM +0000, Jag Raman wrote:
> 
> 
> > On Mar 29, 2022, at 10:48 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > 
> > On Tue, Mar 29, 2022 at 02:12:40PM +0000, Jag Raman wrote:
> >>> On Mar 29, 2022, at 8:35 AM, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >>> On Fri, Mar 25, 2022 at 03:19:41PM -0400, Jagannathan Raman wrote:
> >>>> +void remote_iommu_del_device(PCIDevice *pci_dev)
> >>>> +{
> >>>> +    int pci_bdf;
> >>>> +
> >>>> +    if (!remote_iommu_table.elem_by_bdf || !pci_dev) {
> >>>> +        return;
> >>>> +    }
> >>>> +
> >>>> +    pci_bdf = PCI_BUILD_BDF(pci_bus_num(pci_get_bus(pci_dev)), pci_dev->devfn);
> >>>> +
> >>>> +    qemu_mutex_lock(&remote_iommu_table.lock);
> >>>> +    g_hash_table_remove(remote_iommu_table.elem_by_bdf, INT2VOIDP(pci_bdf));
> >>>> +    qemu_mutex_unlock(&remote_iommu_table.lock);
> >>>> +}
> >>>> +
> >>>> +void remote_configure_iommu(PCIBus *pci_bus)
> >>>> +{
> >>>> +    if (!remote_iommu_table.elem_by_bdf) {
> >>>> +        remote_iommu_table.elem_by_bdf =
> >>>> +            g_hash_table_new_full(NULL, NULL, NULL, remote_iommu_del_elem);
> >>>> +        qemu_mutex_init(&remote_iommu_table.lock);
> >>>> +    }
> >>>> +
> >>>> +    pci_setup_iommu(pci_bus, remote_iommu_find_add_as, &remote_iommu_table);
> >>> 
> >>> Why is remote_iommu_table global? It could be per-PCIBus and indexed by
> >>> just devfn instead of the full BDF.
> >> 
> >> It’s global because remote_iommu_del_device() needs it for cleanup.
> > 
> > Can remote_iommu_del_device() use pci_get_bis(pci_dev)->irq_opaque to
> > get the per-bus table?
> 
> pci_get_bus(pci_dev)->irq_opaque is used for interrupts.
> 
> PCIBus already has an iommu_opaque, which is a private
> member of the bus structure. It’s passed as an argument
> to the iommu_fn().
> 
> We could add a getter function to retrieve PCIBus->iommu_opaque
> in remote_iommu_del_device(). That way we could avoid the global variable.

I've CCed Michael, Peter, and Jason regarding IOMMUs.

This makes me wonder whether there is a deeper issue with the
pci_setup_iommu() API: the lack of per-device cleanup callbacks.
Per-device IOMMU resources should be freed when a device is hot
unplugged.

From what I can tell this is not the case today:

- hw/i386/intel_iommu.c:vtd_find_add_as() allocates and adds device
  address spaces but I can't find where they are removed and freed.
  VTDAddressSpace instances pointed to from vtd_bus->dev_as[] are leaked.

- hw/i386/amd_iommu.c has similar leaks.

Your patch introduces a custom remote_iommu_del_device() function, but I
think the pci_setup_iommu() API should take a device_del() callback so
IOMMUs have a standard interface for handling per-device cleanup.

BTW in your case remote_iommu_del_device() is sufficient because hot
unplug is blocked by the new unplug blocker mechanism you introduced.
For other IOMMUs unplug will not be blocked and therefore IOMMUs really
need a callback for per-device cleanup.

Stefan

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