Re: [RFC v2 18/18] guest memory protection: Alter virtio default properties for protected guests

[David Gibson]

On Sat, Jun 06, 2020 at 04:21:31PM -0400, Michael S. Tsirkin wrote:
> On Thu, May 21, 2020 at 01:43:04PM +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 guest memory protection 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 88d699bceb..cb6580954e 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/guest-memory-protection.h"
> > +#include "hw/virtio/virtio.h"
> > +#include "hw/virtio/virtio-pci.h"
> >  
> >  GlobalProperty hw_compat_5_0[] = {};
> >  const size_t hw_compat_5_0_len = G_N_ELEMENTS(hw_compat_5_0);
> > @@ -1159,6 +1161,15 @@ 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");
> >      }
> >  
> 
> I think it's a reasonable way to address this overall.
> As Cornelia has commented, addressing ccw as well

Sure.  I was assuming somebody who actually knows ccw could do that as
a follow up.

> as cases where user has
> specified the property manually could be worth-while.

I don't really see what's to be done there.  I'm assuming that if the
user specifies it, they know what they're doing - particularly with
nonstandard guests there are some odd edge cases where those
combinations might work, they're just not very likely.

-- 
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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