Re: [for-5.2 v4 09/10] host trust limitation: Alter virtio default properties for protected guests

[Greg Kurz]

On Mon, 27 Jul 2020 16:05:14 +0100
"Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:

> * David Gibson (david@gibson.dropbear.id.au) 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@gibson.dropbear.id.au>
> 
> Good, it's just too easy to forget them at the moment and get hopelessly
> confused.
> 
> 
> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> 
> > ---
> >  hw/core/machine.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > index b599b0ba65..2a723bf07b 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" },
> > @@ -1161,6 +1163,15 @@ 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");

What about non-transitional devices (eg. vhost-user-fs-pci) ? They don't know
about "disable-legacy" since they don't need it.

> > +        object_register_sugar_prop(TYPE_VIRTIO_DEVICE, "iommu_platform", 
> > "on");
> >      }
> >  
> >      machine_class->init(machine);
> > -- 
> > 2.26.2
> > 
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 
>