Re: [for-6.0 v5 12/13] securable guest memory: Alter virtio default properties for protected guests

[Christian Borntraeger]

On 04.12.20 09:17, Cornelia Huck wrote:
> On Fri, 4 Dec 2020 09:10:36 +0100
> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
> 
>> On 04.12.20 06:44, 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 securable guest memory 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>
>>> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
>>> ---
>>>  hw/core/machine.c | 13 +++++++++++++
>>>  1 file changed, 13 insertions(+)
>>>
>>> diff --git a/hw/core/machine.c b/hw/core/machine.c
>>> index a67a27d03c..d16273d75d 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/securable-guest-memory.h"
>>> +#include "hw/virtio/virtio.h"
>>> +#include "hw/virtio/virtio-pci.h"
>>>  
>>>  GlobalProperty hw_compat_5_1[] = {
>>>      { "vhost-scsi", "num_queues", "1"},
>>> @@ -1169,6 +1171,17 @@ 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);  
>>
>> I have not followed all the history (sorry). Should we also set 
>> iommu_platform
>> for virtio-ccw? Halil?
>>
> 
> That line should add iommu_platform for all virtio devices, shouldn't
> it?

Yes, sorry. Was misreading that with the line above.