Circuit for Latching Power Supply

Almost every microcontroller support any kind of sleep mode which enables power saving. But even in sleep mode or deep sleep mode power is consumed. So I wondered if there is a circuit design which can be used for absolutely switching off a controller uppon its request. The circuit shall suport some kind of switch to switch on the controller.
The first question to be answered is: Should you switch the high side or low side of the circuit. Well, in most designs I have some kind of ground plane. The controller is attached to the ground plane with multiple vias. Sometimes because the controller does have multiple ground pins or just to allow a good flow of charges. So switching low side seems not to be a good approach.
A mechanical switch could be a good solution for some projects, but most of the time a transistor is the better way to go. I picked a p-channel MosFET. A IRF9540N to be precise. It is kinda overkill for a microcontroller, but I had it laying around.
Next we need a way to activate or deactivate the MosFET. This will be done by a BJT, like the jellybean BC547 (Q3). It will be used to pull the gate of the MosFET to ground to make it conduct. But how do we switch the BC547?
I decided for a reed switch, but you can use all kind of switches. If the reed switch is closed, the Q3 will pull Q2s gate to ground and power will be switched on.
R6 keeps the circuit latched even if the reed switch is opened. C2 is needed to prevent toggling.
To switch the circuit off we add a second BC547 (Q1). It will be controlled by the microcontroller.

What you have to keep in mind
The continuous collector current of a BC547 shal not exeed 100 mA. So you have to pick the resistors R3 and R4 according to your input voltage. In my case I designed for 12V. R_3/4 = 12V / 0,1A = 120 Ω
I went of 100 kΩ for almost every resistor, except R₃. Because I ran out of 100 kΩ resistors.