• Ingo Molnar's avatar
    x86/boot: Reorganize and clean up the BIOS area reservation code · edce2121
    Ingo Molnar authored
    So the reserve_ebda_region() code has accumulated a number of
    problems over the years that make it really difficult to read
    and understand:
    
    - The calculation of 'lowmem' and 'ebda_addr' is an unnecessarily
      interleaved mess of first lowmem, then ebda_addr, then lowmem tweaks...
    
    - 'lowmem' here means 'super low mem' - i.e. 16-bit addressable memory. In other
      parts of the x86 code 'lowmem' means 32-bit addressable memory... This makes it
      super confusing to read.
    
    - It does not help at all that we have various memory range markers, half of which
      are 'start of range', half of which are 'end of range' - but this crucial
      property is not obvious in the naming at all ... gave me a headache trying to
      understand all this.
    
    - Also, the 'ebda_addr' name sucks: it highlights that it's an address (which is
      obvious, all values here are addresses!), while it does not highlight that it's
      the _start_ of the EBDA region ...
    
    - 'BIOS_LOWMEM_KILOBYTES' says a lot of things, except that this is the only value
      that is a pointer to a value, not a memory range address!
    
    - The function name itself is a misnomer: it says 'reserve_ebda_region()' while
      its main purpose is to reserve all the firmware ROM typically between 640K and
      1MB, while the 'EBDA' part is only a small part of that ...
    
    - Likewise, the paravirt quirk flag name 'ebda_search' is misleading as well: this
      too should be about whether to reserve firmware areas in the paravirt case.
    
    - In fact thinking about this as 'end of RAM' is confusing: what this function
      *really* wants to reserve is firmware data and code areas! Once the thinking is
      inverted from a mixed 'ram' and 'reserved firmware area' notion to a pure
      'reserved area' notion everything becomes a lot clearer.
    
    To improve all this rewrite the whole code (without changing the logic):
    
    - Firstly invert the naming from 'lowmem end' to 'BIOS reserved area start'
      and propagate this concept through all the variable names and constants.
    
    	BIOS_RAM_SIZE_KB_PTR		// was: BIOS_LOWMEM_KILOBYTES
    
    	BIOS_START_MIN			// was: INSANE_CUTOFF
    
    	ebda_start			// was: ebda_addr
    	bios_start			// was: lowmem
    
    	BIOS_START_MAX			// was: LOWMEM_CAP
    
    - Then clean up the name of the function itself by renaming it
      to reserve_bios_regions() and renaming the ::ebda_search paravirt
      flag to ::reserve_bios_regions.
    
    - Fix up all the comments (fix typos), harmonize and simplify their
      formulation and remove comments that become unnecessary due to
      the much better naming all around.
    Signed-off-by: Ingo Molnar's avatarIngo Molnar <mingo@kernel.org>
    edce2121
head32.c 1.22 KB