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

[Cornelia Huck]

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?