On Thu, 21 May 2020 13:43:04 +1000
David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxx>
> ---
> 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);
