On Fri, Jun 19, 2020 at 07:46:10AM -0400, Michael S. Tsirkin 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@xxxxxxxxxxxxxxxxxxxxx>
> > > ---
> > > 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 is a bad idea.
> > The "disable-legacy" option in particular is undesirable as that switches
> > the device to virtio-1.0 only mode, which exposes a different PCI ID to
> > the guest.
> >
> > If some options are incompatible with encryption, then we should raise a
> > fatal error at startup, so applications/admins are aware that their requested
> > config is broken.
> >
> > Regards,
> > Daniel
>
> Agreed - my suggestion is an on/off/auto property, auto value
> changes automatically, on/off is validated.
So, I think you're specifically suggesting this for the
"iommu_platform" property, by delaying determining which mode to use
until the guest activates the device. Is that correct?
That might work on s390, but I don't think it will work on POWER on at
least 2 counts:
1) qemu doesn't actually have a natural way of determining if a guest
is in secure mode (that's handled directly between the guest and
the ultravisor). So even at driver init time, we won't know the
right value.
2) for virtio-pci, iommu_platform=on requires a "modern" device, not a
legacy or transitional one. That changes the PCI ID, which means
we can't delay deciding it until driver init
--
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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