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

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Jun 22, 2020 at 10:09:30AM +0100, Daniel P. Berrangé wrote:
> On Sat, Jun 20, 2020 at 06:24:27PM +1000, David Gibson wrote:
> > On Fri, Jun 19, 2020 at 04:05:56PM +0100, Daniel P. Berrangé wrote:
> > > 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.
> > 
> > Umm.. by this argument we can never change any default, ever.  But we
> > do that routinely with new machine versions.  How is changing based on
> > a machine option different from that?
> 
> It isn't really different. Most of the time we get away with it and no one
> sees a problem. Some of the changes made though, do indeed break things,
> and libvirt tries to override QEMU's changes in defaults where they are
> especially at risk of causing breakage. The virtio device model is one such
> change I'd consider especially risky as there are clear guest OS driver
> support compatibility issues there, with it being a completely different
> PCI device ID & impl.

If it were possible to drop an existing supported guest into secure VM
mode, that would make sense.  But AFAICT, a guest always need to be
aware of the secure mode - it certainly does on POWER.  Plus, support
for secure guest mode is way newer than support for modern virtio
devices.

Even then, I don't see that this is really anything new.  Updating
machine type version can absolutely change system devices in a way
which could break guests (though it mostly won't).  If you really want
stable support for a given guest, use a versioned machine type.  Doing
that will work just as well for the secure VM stuff as for anything
else.

-- 
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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