Hi Sven,
as tom said on IRC - to some extend it is that Bionic didn't have a Rome chip 
definition yet, so the changes to add it to Rome do not 100% make sense there.

You have as part of similar fixes there already:
1. all kind of intel chips which were faster to provide that for 2.11 (in the 
stable tree of qemu)

root@b:~# qemu-system-x86_64 -cpu ? | grep -i ibrs
x86   Broadwell-IBRS  Intel Core Processor (Broadwell, IBRS)          
x86 Broadwell-noTSX-IBRS  Intel Core Processor (Broadwell, no TSX, IBRS)  
x86     Haswell-IBRS  Intel Core Processor (Haswell, IBRS)            
x86 Haswell-noTSX-IBRS  Intel Core Processor (Haswell, no TSX, IBRS)    
x86   IvyBridge-IBRS  Intel Xeon E3-12xx v2 (Ivy Bridge, IBRS)        
x86     Nehalem-IBRS  Intel Core i7 9xx (Nehalem Core i7, IBRS update)
x86 SandyBridge-IBRS  Intel Xeon E312xx (Sandy Bridge, IBRS update)   
x86 Skylake-Client-IBRS  Intel Core Processor (Skylake, IBRS)            
x86 Skylake-Server-IBRS  Intel Xeon Processor (Skylake, IBRS)            
x86    Westmere-IBRS  Westmere E56xx/L56xx/X56xx (IBRS update)

2. a generic EPYC chip which got what AMD first pushed for that was IBPB
(mind all the characters - not the same)

root@b:~# qemu-system-x86_64 -cpu ? | grep -i epy
x86        EPYC-IBPB  AMD EPYC Processor (with IBPB)                  
x86             EPYC  AMD EPYC Processor

I rechecked if e.g. upstream stable of 2.11 since then changed things,
but there is nothing in this regard. Nor did I see (in a very quick not
thorough check) anyone else backporting it that far.

So you might say "can't I use shiny new chips and all of their feature
with Bionic :-/", but for some applications stacks this is a common
issue (the balance between stability vs staying new). And for those
there often is a way between the obvious first recommendation "use a
newer LTS and you get the newer bits" in the form of
https://wiki.ubuntu.com/OpenStack/CloudArchive or
https://launchpad.net/~canonical-server/+archive/ubuntu/server-backports
that allow you to get some newer stacks for older LTSes.

Back then when this bug came up these patches did neither make sense nor
where in any way applicable to Bionic, one could take some more time to
deeply re-evaluate this, but as I said above there are options already
...; I'm tempted to say that this should be spawned into a separate bug
then to keep this one as-is reflecting only the efforts of the past.

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https://bugs.launchpad.net/bugs/1921754

Title:
  Add missing cpu feature bits in EPYC-Rome model

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