[Qemu-devel] [PATCH 0/6] target-mips: add MDI semihosting

[Nathan Froyd <froydnj@codesourcery.com>]

This patch series adds semihosting for bare-metal 32-bit MIPS targets
that follows the MDI semihosting model used in MIPS's development
product.

The semihosting protocol works as follows:

- All semihosted calls go through _mdi_syscall, which in the absence of
  simulator support, returns ENOSYS;

- The address of _mdi_syscall is stored in a special section, .sdeosabi;

- When an MDI semihosting-aware simulator is used, a breakpoint is
  placed at _mdi_syscall;

- When this breakpoint is hit, the simulator performs the requested
  action and returns control to the program.

Discovering the address of _mdi_syscall is straightforward when the
program is passed in via -kernel.  We extended the ELF loader interface
slightly so that the caller could inspect the sections of the binary and
act on the contents of the sections if they so chose.  Things are
slightly trickier when using QEMU as a GDB remote target, as the program
will not be provided via -kernel, but via `load' from within GDB.  This
second method requires that we implement qSymbol support in the GDB
stub so we can directly ask GDB where _mdi_syscall is.

The patches have been built for {mips,mips64}-softmmu targets and tested
with the libstdc++ testsuite (which exercises things like read(),
write(), and lseek()).

-Nathan