Re: [PATCH v8 12/13] confidential guest support: Alter virtio default properties for protected guests

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Feb 02, 2021 at 06:06:34PM -0500, Michael S. Tsirkin wrote:
> On Tue, Feb 02, 2021 at 03:13:14PM +1100, 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 confidential guest 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>
> > Reviewed-by: Cornelia Huck <cohuck@redhat.com>
> > Reviewed-by: Greg Kurz <groug@kaod.org>
> 
> 
> > ---
> >  hw/core/machine.c | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> > 
> > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > index 94194ab82d..497949899b 100644
> > --- a/hw/core/machine.c
> > +++ b/hw/core/machine.c
> > @@ -33,6 +33,8 @@
> >  #include "migration/global_state.h"
> >  #include "migration/vmstate.h"
> >  #include "exec/confidential-guest-support.h"
> > +#include "hw/virtio/virtio.h"
> > +#include "hw/virtio/virtio-pci.h"
> >  
> >  GlobalProperty hw_compat_5_2[] = {};
> >  const size_t hw_compat_5_2_len = G_N_ELEMENTS(hw_compat_5_2);
> > @@ -1196,6 +1198,17 @@ 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);
> 
> So overriding a boolean property always poses a problem:
> if user does set iommu_platform=off we are ignoring this
> silently.

No, we don't.  That's why this is register_sugar_prop() rather than an
outright set_prop().  An explicitly given option will take precedence.

> Can we change iommu_platform to on/off/auto? Then we can
> change how does auto behave.

I've never had a satisfactory explanation of what the semantics of
"auto" need to be.

> 
> Bonus points for adding "access_platform" and making it
> a synonym of platform_iommu.
> 
> >      }
> >  
> >      machine_class->init(machine);
> 

-- 
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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