Hi Dee,
You may have already seen that CICS/TS 5.4 is stated as supported with Gen 8.5 on these compatibility links:
https://support.ca.com/phpdocs/0/MSPSaccount/COMPAT/cics_compat.HTML
https://support.ca.com/phpdocs/0/MSPSaccount/COMPAT/GEN.HTML
So far I could not find any problems reported by other users for Gen 8.5 (or 8.6) with CICS/TS 5.4.
For the concurrency setting of THREADSAFE v QUASIRENT there is a Threadsafe section of the Gen 8.5 docops:
https://docops.ca.com/ca-gen/8-5/en/installing/install-ca-gen-on-z-os/configuring-runtime-ims-and-cics/threadsafe
As you can see it states "Applications that access DB2 resources and are defined as quasirent must do TCB switching. TCB switching impacts application performance.". Using THREADSAFE is the alternative.
Specifically for DSNTIAC I checked further with Engineering:
DSNTIAC as provided by IBM is not a DLL and as such it cannot be directly called from Gen DLL applications. Gen normally calls DSNTIAC via the TIRCTIAX user exit (TIRCTIAZ DLL). However DSNTIAC (or TIRCTIAX) are only invoked when there is a DB2 error to format the displayed message and hopefully you are not getting too many of those, so in this case even using QUASIRENT should not cause excessive TCB switching.
For your testing they suggested you decide how you want to use DSNTIAC (dynamic vs static, default in TIRCTIAZ is to dynamic call of DSNTIAR) and define it as THREADSAFE.
https://docops.ca.com/ca-gen/8-6/en/reference/user-exits/z-os-user-exits/z-os-server-user-exits-cics/tirtiarx-db2-message-exits
NOTE: Of much more relevance is the DB2 Connection Dynamic Plan Exit program TIRC$EXT documented on the Threadsafe page. This program is invoked for every transaction, particularly if DB2 Dynamic Pool is used, and it does affect TCB switching. So it is suggested you should look if the definition of TIRC$EXT (especially if it is specified in your CICS DB2CONN) is THREADSAFE.
If you want to go ahead with testing Gen 8.5 with CICS/TS 5.4 using above concurrency suggestions and report any problems via support cases we can work through them.
Alternatively you can wait for Engineering to run some tests but due to the holiday season that may take a few weeks.
Hope that helps
Regards,
Lynn