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

[Cornelia Huck <cohuck@redhat.com>]

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On Tue, 9 Jun 2020 17:40:46 +0200
Halil Pasic <pasic@linux.ibm.com> wrote:

> On Tue, 9 Jun 2020 12:16:41 +0200
> Cornelia Huck <cohuck@redhat.com> wrote:
> 
> > On Sun, 7 Jun 2020 13:07:35 +1000
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> >   
> > > 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.  
> > 
> > FWIW, I think we could simply enable iommu_platform for protected
> > guests for ccw; no prereqs like pci's disable-legacy.
> >   
> 
> For s390x having a memory-encryption object is not prereq for doing
> protected virtualization, so the scheme does not work for us right now.

Yeah, that would obviously need to be added first.

> 
> I hope Jansoch will chime in after he is back from his vacation. IMHO
> having a memory-protection object will come in handy for migration,
> but the presence or absence of this object should be largely transparent
> to the user (and not something that needs to be explicitly managed via
> command line). AFAIU this object is in the end it is just QEMU plumbing.
> 
> > >   
> > > > 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.  
> > 
> > If I understood Halil correctly, devices without iommu_platform
> > apparently can crash protected guests on s390. Is that supposed to be a
> > "if it breaks, you get to keep the pieces" situation, or do we really
> > want to enforce iommu_platform?  
> 
> I strongly oppose to adopting the "if it breaks, you get to keep the
> pieces" strategy here. It is borderline acceptable on startup, although
> IMHO not preferable, but a device hotplug bringing down a guest that is
> already running userspace is not acceptable at all.

There's still the option to fail to add such a device, though.

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