hw/virtio/virtio-iommu-pci: Enforce the device is plugged on the root bus

In theory the virtio-iommu-pci could be plugged anywhere in the PCIe
topology and as long as the dt/acpi info are properly built this should
work. However at the moment we fail to do that because the
virtio-iommu-pci BDF is not computed at plug time and in that case
vms->virtio_iommu_bdf gets an incorrect value.

For instance if the virtio-iommu-pci is plugged onto a pcie root port
and the virtio-iommu protects a virtio-block-pci device the guest does
not boot.

So let's do not pretend we do support this case and fail the initialize()
if we detect the virtio-iommu-pci is plugged anywhere else than on the
root bus. Anyway this ability is not needed.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
---
 hw/virtio/virtio-iommu-pci.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

On Wed, Oct 12, 2022 at 12:34:48PM -0400, Eric Auger wrote:
> In theory the virtio-iommu-pci could be plugged anywhere in the PCIe
> topology and as long as the dt/acpi info are properly built this should
> work. However at the moment we fail to do that because the
> virtio-iommu-pci BDF is not computed at plug time and in that case
> vms->virtio_iommu_bdf gets an incorrect value.
> 
> For instance if the virtio-iommu-pci is plugged onto a pcie root port
> and the virtio-iommu protects a virtio-block-pci device the guest does
> not boot.
> 
> So let's do not pretend we do support this case and fail the initialize()
> if we detect the virtio-iommu-pci is plugged anywhere else than on the
> root bus. Anyway this ability is not needed.
> 
> Signed-off-by: Eric Auger <eric.auger@redhat.com>

I agree with the reasoning. It's not supported, difficult to support and
not needed, so let's make the error more obvious. If I remember correctly,
the problem with supporting an IOMMU behind a bridge is that static
topology descriptions like DT and ACPI identify devices using bus numbers,
which QEMU would need to allocate.

> ---
>  hw/virtio/virtio-iommu-pci.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/virtio/virtio-iommu-pci.c b/hw/virtio/virtio-iommu-pci.c
> index 79ea8334f0..7ef2f9dcdb 100644
> --- a/hw/virtio/virtio-iommu-pci.c
> +++ b/hw/virtio/virtio-iommu-pci.c
> @@ -17,6 +17,7 @@
>  #include "hw/qdev-properties-system.h"
>  #include "qapi/error.h"
>  #include "hw/boards.h"
> +#include "hw/pci/pci_bus.h"
>  #include "qom/object.h"
>  
>  typedef struct VirtIOIOMMUPCI VirtIOIOMMUPCI;
> @@ -44,6 +45,7 @@ static Property virtio_iommu_pci_properties[] = {
>  static void virtio_iommu_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
>  {
>      VirtIOIOMMUPCI *dev = VIRTIO_IOMMU_PCI(vpci_dev);
> +    PCIBus *pbus = pci_get_bus(&vpci_dev->pci_dev);
>      DeviceState *vdev = DEVICE(&dev->vdev);
>      VirtIOIOMMU *s = VIRTIO_IOMMU(vdev);
>  
> @@ -57,11 +59,17 @@ static void virtio_iommu_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
>              s->reserved_regions[i].type != VIRTIO_IOMMU_RESV_MEM_T_MSI) {
>              error_setg(errp, "reserved region %d has an invalid type", i);
>              error_append_hint(errp, "Valid values are 0 and 1\n");
> +            return;

Should this be a separate fix?  Anyway, for both changes

Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Tested-by: Jean-Philippe Brucker <jean-philippe@linaro.org>

>          }
>      }
> +    if (!pci_bus_is_root(pbus)) {
> +        error_setg(errp, "virtio-iommu-pci must be plugged on the root bus");
> +        return;
> +    }
> +
>      object_property_set_link(OBJECT(dev), "primary-bus",
> -                             OBJECT(pci_get_bus(&vpci_dev->pci_dev)),
> -                             &error_abort);
> +                             OBJECT(pbus), &error_abort);
> +
>      virtio_pci_force_virtio_1(vpci_dev);
>      qdev_realize(vdev, BUS(&vpci_dev->bus), errp);
>  }
> -- 
> 2.31.1
>

On Wed, Oct 12, 2022 at 12:34:48PM -0400, Eric Auger wrote:
> In theory the virtio-iommu-pci could be plugged anywhere in the PCIe
> topology and as long as the dt/acpi info are properly built this should
> work. However at the moment we fail to do that because the
> virtio-iommu-pci BDF is not computed at plug time and in that case
> vms->virtio_iommu_bdf gets an incorrect value.
>
> For instance if the virtio-iommu-pci is plugged onto a pcie root port
> and the virtio-iommu protects a virtio-block-pci device the guest does
> not boot.
>
> So let's do not pretend we do support this case and fail the initialize()
> if we detect the virtio-iommu-pci is plugged anywhere else than on the
> root bus. Anyway this ability is not needed.
>
> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> ---
>  hw/virtio/virtio-iommu-pci.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)

FYI libvirt will already reject any attempts to place the device
anywhere but directly on pcie.0, so from our point of view merging
this patch is perfectly fine.