On Tue, 12 Jan 2021 15:45:07 +1100
David Gibson <da...@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 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 <da...@gibson.dropbear.id.au>
> Reviewed-by: Dr. David Alan Gilbert <dgilb...@redhat.com>
> Reviewed-by: Cornelia Huck <coh...@redhat.com>
> ---
Reviewed-by: Greg Kurz <gr...@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 5a7433332b..61ae363559 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);
> @@ -1204,6 +1206,17 @@ 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);
> }
>
> machine_class->init(machine);
