On Fri, Jun 19, 2020 at 12:06:02PM +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 host trust limitation 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@xxxxxxxxxxxxxxxxxxxxx>
> ---
> hw/core/machine.c | 11 +++++++++++
> 1 file changed, 11 insertions(+)
>
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index a71792bc16..8dfc1bb3f8 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/host-trust-limitation.h"
> +#include "hw/virtio/virtio.h"
> +#include "hw/virtio/virtio-pci.h"
>
> GlobalProperty hw_compat_5_0[] = {
> { "virtio-balloon-device", "page-poison", "false" },
> @@ -1165,6 +1167,15 @@ 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");
> }
Silently changing the user's request configuration like this is a bad idea.
The "disable-legacy" option in particular is undesirable as that switches
the device to virtio-1.0 only mode, which exposes a different PCI ID to
the guest.
If some options are incompatible with encryption, then we should raise a
fatal error at startup, so applications/admins are aware that their requested
config is broken.
Regards,
Daniel
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