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@xxxxxxxxxx> wrote:
>
> > On 04.12.20 09:17, Cornelia Huck wrote:
> > > On Fri, 4 Dec 2020 09:10:36 +0100
> > > Christian Borntraeger <borntraeger@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxx>
> > >>> Reviewed-by: Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx>
> > >>> ---
> > >>> 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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