46cd2c1050f0 ("hw/tpm: add PPI support to tpm-tis-device for ARM64
virt") unconditionally registers a "tpm-ppi" RAMBlock when the TPM
TIS sysbus device is realized.  This breaks backward migration: a
QEMU with PPI support cannot migrate to an older QEMU without it:

  Unknown ramblock "tpm-ppi", cannot accept migration

The failure is 100% reproducible on any aarch64 guest that uses
tpm-tis-device when migrating to a host whose QEMU predates the PPI
commit.

Fix by gating the PPI memory region behind a new "ppi" device
property, and disabling it via hw_compat_11_0[] for machine types
<= virt-11.0.

Mohammadfaiz Bawa (2):
  hw/tpm: gate PPI support on tpm-tis-device behind a device property
  hw/core/machine: disable tpm-tis-device PPI for machine type <= 11.0

 hw/core/machine.c       |  1 +
 hw/tpm/tpm_tis.h        |  1 +
 hw/tpm/tpm_tis_sysbus.c | 16 +++++++++++-----
 3 files changed, 13 insertions(+), 5 deletions(-)

-- 
2.54.0