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

[Daniel P . Berrangé]

On Sat, Jun 20, 2020 at 12:45:41AM +1000, David Gibson wrote:
> 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.

Don't assume that the user has set "disable-legacy=off". People who want to
have a transtional device are almost certainly pasing "-device virtio-blk-pci",
because historical behaviour is that this is sufficient to give you a
transitional device. Changing the default of disable-legacy=on has not
honoured the users' requested config.

Regards,
Daniel
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