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

[David Gibson]

On Fri, Dec 04, 2020 at 03:43:10PM +0100, Halil Pasic wrote:
> On Fri, 4 Dec 2020 09:29:59 +0100
> Christian Borntraeger <borntraeger@de.ibm.com> wrote:
> 
> > 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. 
> > 
> 
> I believe this is the best we can get. In a sense it is still a
> pessimization,

I'm not really clear on what you're getting at here.

> but it is a big usability improvement compared to having
> to set iommu_platform manually. 
> 
> Regards,
> Halil
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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