Old:
qemu 1:4.2-3ubuntu6.15, libvirt 6.0.0-0ubuntu8.8

# qemu-system-x86_64 -cpu ? | grep Rome
x86 EPYC-Rome             (alias configured by machine type)              
x86 EPYC-Rome-v1          AMD EPYC-Rome Processor

# virsh domcapabilities
    <mode name='host-model' supported='yes'>
      <model fallback='forbid'>EPYC-Rome</model>
      <vendor>AMD</vendor>
      <feature policy='require' name='x2apic'/>
      <feature policy='require' name='tsc-deadline'/>
      <feature policy='require' name='hypervisor'/>
      <feature policy='require' name='tsc_adjust'/>
      <feature policy='require' name='spec-ctrl'/>
      <feature policy='require' name='stibp'/>
      <feature policy='require' name='arch-capabilities'/>
      <feature policy='require' name='ssbd'/>
      <feature policy='require' name='xsaves'/>
      <feature policy='require' name='cmp_legacy'/>
      <feature policy='require' name='invtsc'/>
      <feature policy='require' name='amd-ssbd'/>
      <feature policy='require' name='virt-ssbd'/>
      <feature policy='require' name='rdctl-no'/>
      <feature policy='require' name='skip-l1dfl-vmentry'/>
      <feature policy='require' name='mds-no'/>
      <feature policy='require' name='pschange-mc-no'/>
    </mode>

New:
# qemu-system-x86_64 -cpu ? | grep Rome
x86 EPYC-Rome             (alias configured by machine type)              
x86 EPYC-Rome-v1          AMD EPYC-Rome Processor                         
x86 EPYC-Rome-v2          AMD EPYC-Rome Processor

# virsh domcapabilities
    <mode name='host-model' supported='yes'>
      <model fallback='forbid'>EPYC-Rome</model>
      <vendor>AMD</vendor>
      <feature policy='require' name='x2apic'/>
      <feature policy='require' name='tsc-deadline'/>
      <feature policy='require' name='hypervisor'/>
      <feature policy='require' name='tsc_adjust'/>
      <feature policy='require' name='spec-ctrl'/>
      <feature policy='require' name='stibp'/>
      <feature policy='require' name='arch-capabilities'/>
      <feature policy='require' name='ssbd'/>
      <feature policy='require' name='xsaves'/>
      <feature policy='require' name='cmp_legacy'/>
      <feature policy='require' name='invtsc'/>
      <feature policy='require' name='ibrs'/>
      <feature policy='require' name='amd-ssbd'/>
      <feature policy='require' name='virt-ssbd'/>
      <feature policy='require' name='rdctl-no'/>
      <feature policy='require' name='skip-l1dfl-vmentry'/>
      <feature policy='require' name='mds-no'/>
      <feature policy='require' name='pschange-mc-no'/>
    </mode>

Instance with ibrs flag can now be run

processor       : 0
vendor_id       : AuthenticAMD
cpu family      : 23
model           : 49
model name      : AMD EPYC-Rome Processor
stepping        : 0
microcode       : 0x1000065
cpu MHz         : 2495.312
cache size      : 512 KB
physical id     : 0
siblings        : 2
core id         : 0
cpu cores       : 2
apicid          : 0
initial apicid  : 0
fpu             : yes
fpu_exception   : yes
cpuid level     : 13
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm rep_good nopl cpuid extd_apicid tsc_known_freq pni pclmulqdq ssse3 fma cx16 sse4_1 sse4_2 x2apic movbe popcnt aes xsave avx f16c rdrand hypervisor lahf_lm cmp_legacy svm cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw topoext perfctr_core ssbd ibrs ibpb stibp vmmcall fsgsbase bmi1 avx2 smep bmi2 rdseed adx smap clflushopt clwb sha_ni xsaveopt xsavec xgetbv1 xsaves clzero xsaveerptr wbnoinvd arat npt nrip_save umip rdpid
bugs            : sysret_ss_attrs spectre_v1 spectre_v2 spec_store_bypass
bogomips        : 4990.62
TLB size        : 1024 4K pages
clflush size    : 64
cache_alignment : 64
address sizes   : 40 bits physical, 48 bits virtual
power management:


Migration of guest (e.g. with EPYC-IBPB or EPYC-Rome) from previous version to proposed still works.

Migration of guest (EPYC-Rome with ibrs) to previous version doesn't (as expected)