Re: [for-5.2 v4 09/10] host trust limitation: Alter virtio default properties for protected guests

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* David Gibson (david@gibson.dropbear.id.au) wrote:
> The default behaviour for virtio devices is not to use the platforms normal
> DMA paths, but instead to use the fact that it's running in a hypervisor
> to directly access guest memory.  That doesn't work if the guest's memory
> is protected from hypervisor access, such as with AMD's SEV or POWER's PEF.
> 
> So, if a host trust limitation mechanism is enabled, then apply the
> iommu_platform=on option so it will go through normal DMA mechanisms.
> Those will presumably have some way of marking memory as shared with the
> hypervisor or hardware so that DMA will work.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Good, it's just too easy to forget them at the moment and get hopelessly
confused.


Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
>  hw/core/machine.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index b599b0ba65..2a723bf07b 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -28,6 +28,8 @@
>  #include "hw/mem/nvdimm.h"
>  #include "migration/vmstate.h"
>  #include "exec/host-trust-limitation.h"
> +#include "hw/virtio/virtio.h"
> +#include "hw/virtio/virtio-pci.h"
>  
>  GlobalProperty hw_compat_5_0[] = {
>      { "virtio-balloon-device", "page-poison", "false" },
> @@ -1161,6 +1163,15 @@ void machine_run_board_init(MachineState *machine)
>           * areas.
>           */
>          machine_set_mem_merge(OBJECT(machine), false, &error_abort);
> +
> +        /*
> +         * Virtio devices can't count on directly accessing guest
> +         * memory, so they need iommu_platform=on to use normal DMA
> +         * mechanisms.  That requires disabling legacy virtio support
> +         * for virtio pci devices
> +         */
> +        object_register_sugar_prop(TYPE_VIRTIO_PCI, "disable-legacy", "on");
> +        object_register_sugar_prop(TYPE_VIRTIO_DEVICE, "iommu_platform", "on");
>      }
>  
>      machine_class->init(machine);
> -- 
> 2.26.2
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK