John Reiser wrote: > >>qemu-0.9.0 ... > >>emulating Debian 2.6.18-4-qemu mipsel ... > >>errs when gdb 6.4.90-debian (running on the emulated mipsel) > >>single-steps the user-mode instruction ... > > > This looks like another instance of "Qemu/MIPS doesn't handle > > self-modifying code correctly" (the break instructions inserted > > by gdb are exactly this). > > No, the usage by gdb does *not* qualify as "self-modifying code."
In the context of Qemu system emulation it does... > gdb uses the system call ptrace(PTRACE_POKETEXT, pid, addr, data) > to have the emulated operating system kernel itself modify the memory > of the child process. ... since "child processes" etc. run by the guest kernel are just a foreign thing to Qemu. > Nobody has to guess or to "snoop" the memory > bus in order to discover that the instruction stream is being modified. > Instead, there is direct notification of what is happening. The Linux kernel happily does cache flushes, and Qemu happily ignores them, since it doesn't implement a cache model. (A cache model is not the answer. It would be slow, it would only paper over the problem, it wouldn't help for uncached accesses or cacheless systems). > If nothing > else, then under CONFIG_QEMU the implementation of sys_ptrace() > must notify the emulator to flush the appropriate translations. Hacking special facilities in the guest kernel just to work around a Qemu bug is IMHO the wrong approach. Thiemo
