Re: [RFC v2 18/18] guest memory protection: Alter virtio default properties for protected guests

[Cornelia Huck]

On Thu, 21 May 2020 13:43:04 +1000
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 guest memory protection 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.

cc: Halil, who had been looking at the interaction of virtio-ccw
devices and s390 protected virt.

(IIRC, we wanted to try with a on/off/auto property for virtio-ccw?)

> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  hw/core/machine.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index 88d699bceb..cb6580954e 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/guest-memory-protection.h"
> +#include "hw/virtio/virtio.h"
> +#include "hw/virtio/virtio-pci.h"
>  
>  GlobalProperty hw_compat_5_0[] = {};
>  const size_t hw_compat_5_0_len = G_N_ELEMENTS(hw_compat_5_0);
> @@ -1159,6 +1161,15 @@ 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);