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

[David Gibson]

On Tue, Jun 09, 2020 at 12:16:41PM +0200, Cornelia Huck 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.

Right, and the code above should in fact already do so, since it
applies that to TYPE_VIRTIO_DEVICE, which is common.  The
disable-legacy part should be harmless for s390, since this is
effectively just setting a default, and we don't expect any
TYPE_VIRTIO_PCI devices to be instantiated on z.

> > > 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 actually think "if you broke it, keep the pieces" is an acceptable
approach here, but that doesn't preclude some further enforcement to
improve UX.

-- 
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

signature.asc
Description: PGP signature