Re: [PATCH 5/5] hw/arm/virt: Let the virtio-iommu bypass MSIs

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

On Thu, May 07, 2020 at 04:32:01PM +0200, Eric Auger wrote:
> At the moment the virtio-iommu translates MSI transactions.
> This behavior is inherited from ARM SMMU. The virt machine
> code knows where the guest MSI doorbells are so we can easily
> declare those regions as VIRTIO_IOMMU_RESV_MEM_T_MSI. With that
> setting the guest will not map MSIs through the IOMMU and those
> transactions will be simply bypassed. The ITS does MSI translation
> anyway.

Makes sense. I think we have to check that the ITS is used before
declaring the resv region, though. When using gicv2m, the guest will see
this MSI region and deduce that it doesn't need to create a SW mapping for
MSIs (might be wrong, I haven't tested it yet).

> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> ---
>  hw/arm/virt.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index 171e6908ec..6a041e97a5 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -2138,6 +2138,11 @@ static void virt_machine_device_pre_plug_cb(HotplugHandler *hotplug_dev,
>  {
>      if (object_dynamic_cast(OBJECT(dev), TYPE_PC_DIMM)) {
>          virt_memory_pre_plug(hotplug_dev, dev, errp);
> +    } else if (object_dynamic_cast(OBJECT(dev), TYPE_VIRTIO_IOMMU_PCI)) {
> +        /* we declare a VIRTIO_IOMMU_RESV_MEM_T_MSI region */
> +        qdev_prop_set_uint32(dev, "len-reserved-regions", 1);
> +        qdev_prop_set_string(dev, "reserved-regions[0]",
> +                             "0x8090000, 0x809FFFF, 1");

Maybe worth adding a comment saying this is the GITS_TRANSLATER page?

Thanks,
Jean