Re: [PATCH v8 12/13] confidential guest support: Alter virtio default properties for protected guests

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Feb 02, 2021 at 03:13:14PM +1100, David Gibson 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 confidential guest 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>
> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Reviewed-by: Cornelia Huck <cohuck@redhat.com>
> Reviewed-by: Greg Kurz <groug@kaod.org>


> ---
>  hw/core/machine.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index 94194ab82d..497949899b 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -33,6 +33,8 @@
>  #include "migration/global_state.h"
>  #include "migration/vmstate.h"
>  #include "exec/confidential-guest-support.h"
> +#include "hw/virtio/virtio.h"
> +#include "hw/virtio/virtio-pci.h"
>  
>  GlobalProperty hw_compat_5_2[] = {};
>  const size_t hw_compat_5_2_len = G_N_ELEMENTS(hw_compat_5_2);
> @@ -1196,6 +1198,17 @@ 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 also disabling legacy virtio
> +         * support for those virtio pci devices which allow it.
> +         */
> +        object_register_sugar_prop(TYPE_VIRTIO_PCI, "disable-legacy",
> +                                   "on", true);
> +        object_register_sugar_prop(TYPE_VIRTIO_DEVICE, "iommu_platform",
> +                                   "on", false);

So overriding a boolean property always poses a problem:
if user does set iommu_platform=off we are ignoring this
silently.

Can we change iommu_platform to on/off/auto? Then we can
change how does auto behave.

Bonus points for adding "access_platform" and making it
a synonym of platform_iommu.

>      }
>  
>      machine_class->init(machine);
> -- 
> 2.29.2