Re: [PATCH v3 9/9] host trust limitation: Alter virtio default properties for protected guests

[David Gibson]

On Fri, Jun 19, 2020 at 11:12:45AM +0100, Daniel P. Berrangé wrote:
> On Fri, Jun 19, 2020 at 12:06:02PM +1000, 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 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>
> > ---
> >  hw/core/machine.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > index a71792bc16..8dfc1bb3f8 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" },
> > @@ -1165,6 +1167,15 @@ 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");
> > +        object_register_sugar_prop(TYPE_VIRTIO_DEVICE, "iommu_platform", 
> > "on");
> >      }
> 
> Silently changing the user's request configuration like this

It doesn't, though.  register_sugar_prop() effectively registers a
default, so if the user has explicitly specified something, that will
take precedence.

> is a bad idea.
> The "disable-legacy" option in particular is undesirable as that switches
> the device to virtio-1.0 only mode, which exposes a different PCI ID to
> the guest.
> 
> If some options are incompatible with encryption, then we should raise a
> fatal error at startup, so applications/admins are aware that their requested
> config is broken.
> 
> Regards,
> Daniel

-- 
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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