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    1: # [[!template id=man name=uvm_hotplug section=9]] port-masters' FAQ.
    2: 
    3:   1. Why does it matter?
    4: 
    5:      As a port maintainer any architecture that needs to use the virtual memory
    6:      sub-system of NetBSD aka uvm is affected by the
    7:      [[!template id=man name=uvm_hotplug section=9]] change.
    8: 
    9:      Early boot code of every port is affected. Global variables:
   10: 
   11:      	   struct vm_physseg vm_physmem;
   12:      	   int vm_nphysmem;
   13: 
   14:      are no longer visible. They need to be replaced by appropriate
   15:      accessor calls in [[!template id=man name=uvm_hotplug section=9]]
   16: 
   17:      These calls are documented as "Utility Functions" in the
   18:      [[!template id=man name=uvm_hotplug section=9]] manual.
   19: 
   20:      The "switchover" CVS commit log is here:
   21:      <http://mail-index.netbsd.org/source-changes/2016/12/23/msg080110.html>
   22: 
   23:   2. What files may be affected?
   24: 
   25:      In most of the architectures the `sys/arch/<arch_name>/<arch_name>/machdep.c`
   26:      and `sys/arch/<arch_name>/<arch_name>/pmap.c` if they exist are usually
   27:      affected.
   28: 
   29:      But this may not be a exhaustive list. Any other files that deals with pmap and
   30:      stealing pages might also be affected.
   31: 
   32:   3. What does it do ?
   33: 
   34:      [[!template id=man name=uvm_hotplug section=9]] manages the previously exposed `vm_physmem` static array which
   35:      used to keep track of the memory segments.
   36: 
   37:      In the current implementation, the array has been replaced with a
   38:      [[!template id=man name=rbtree section=3]] backing which makes the
   39:      data structure dynamic.
   40: 
   41:      An array based implementation is also provided, for backwards
   42:      compatibility. This is the default implementation and does not provide
   43:      hot pluggability. It is also used without `options UVM_HOTPLUG`
   44:      However the API itself is implementation agnostic.
   45: 
   46:   4. Why is it needed?
   47: 
   48:      With the [[!template id=man name=rbtree section=3]] backing
   49:      implementation, the list of physical pages 
   50:      in the system is no longer in a static array and can dynamically
   51:      expand or collapse, hence adding new pages to the freelist from newly
   52:      available RAM / physical memory (plug) or removing retired pages
   53:      (unplug) via taking them off the freelist and the old vm_physmem.
   54: 
   55:   5. What should I do?
   56: 
   57:      Review the changes to your port due to this new feature. The changes
   58:      may have been made without direct knowledge of your port architecture.
   59: 
   60:      See if your port has hotpluggable hardware. If it does, write a driver
   61:      to use the [[!template id=man name=uvm_hotplug section=9]] api.
   62: 
   63:      An example of [[!template id=man name=uvm_hotplug section=9]] api's
   64:      application can be found in
   65:      [[!template id=man name=balloon section=4]].

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