[Qemu-devel] Can not read SMI handler code with cpu_memory_rw_debug while in SMM

[poletaev]

I am using OVMF2018 
(https://github.com/tianocore/tianocore.github.io/wiki/OVMF) build with smm 
functionality. Run qemu with "-machine smm=on" option.
When SMI handler starts working, cpu_memory_rw_debug() function reads from 
tseg_blackhole device mem and function returns ffff... instead of actual data.
Same situation when using remote gdb debugging, guess because 
cpu_memory_rw_debug() is in use also.
But code of SMI handler executes normally, so cpu can access actual data and 
not tseg_blackhole.

Can anybody explain please, what is the difference in this context between, for 
example, x86_ldub_code() and  cpu_memory_rw_debug()?
Is such behavior intended? (guess no)
What is tseg at all? I can't find similar terminology in intel manuals.