All calls (but one) to kernel.Panic have been replaced by calls to
panic. A call to kernel.Panic is still required to prevent the compiler
from treating kernel.Panic as dead code and eliminating it.
This vmm package exports ReservedZeroedFrame which can be used to setup
a lazy physical page allocation scheme. This is implemented by mapping
ReservedZeroedFrame to each page in a virtual memory region using the
following flag combination: FlagPresent | FlagCopyOnWrite.
This has the effect that all reads from the virtual address region
target the contents of ReservedZeroedFrame (always returning zero). On
the other hand, writes to the virtual address region trigger a page
fault which is resolved as follows:
- a new physical frame is allocated and the contents of ReservedZeroedFrame
are copied to it (effectively clearing the new frame).
- the page entry for the virtual address that caused the fault is
updated to point to the new frame and its flags are changed to:
FlagPresent | FlagRW
- execution control is returned back to the code that caused the fault
Page faults occurring on RO pages with the CopyOnWrite flag set will be
handled by the page handler as follows:
- allocate new frame
- establish temporary mapping for new frame
- copy original page to new frame
- update entry for the page where the fault occurred:
- set physical frame address to the allocated frame
- clear CoW flag and set Present, RW flags
- return from the fault handler to resume execution at the instruction
that caused the fault
Any other page faults will still cause a kernel panic
