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

[Michael S. Tsirkin]

On Thu, May 21, 2020 at 01:43:04PM +1000, 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 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.
> 
> 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");
>      }
>  

I think it's a reasonable way to address this overall.
As Cornelia has commented, addressing ccw as well as cases where user has
specified the property manually could be worth-while.

>      machine_class->init(machine);
> -- 
> 2.26.2