In order to add your own machine, you need at least create one source file, containing the parameters and peripherals of your machine. After that, add a entry inside Makefile.objs, under qemu/hw/arm/. STM32 P103 machine entry.
Let’s take as example Olimex STM32 P103 Development Board: Olimex STM32 P103 Development Board code. In lines 105 and 106, we have flash_size and ram_size. In lines 114 and 115 the code add a LED connect to GPIO A pin 0. In line 130 we have machine description, “Olimex STM32 p103 Dev Board”. In line 131, the machine init function: stm32_p103_init. Another example of a machine more complete: Pebble machine code.
About peripherals, they are instantiated in each family code, considering stm32 case. stm32f1 family: stm32f1xx.c, stm32f2 family: stm32f2xx.c, stm32f4 family: stm32f4xx.c. The peripheral itself is implemented in a driver which typically has a suggestive name: stm32f2xx_adc.c, stm32f2xx_crc.c and so on. Example of a patch that add new peripheral: Addition of ADC to STM32.