Re: [Qemu-devel] [PATCH pic32 2/7] Stop simulation when processor is suspended forever by WAIT instruction with interrupts disabled.

[Christopher Covington <cov@codeaurora.org>]

On 06/30/2015 09:57 PM, Serge Vakulenko wrote:
> Hi Peter and Leon,
> 
> With a bit of thinking, I agree, that the question of session
> termination on WAIT instruction is quite complicated in case of
> multi-core system, background i/o activity, mipsR6 core etc. So I'm
> going to find another solution for the task. What I essentially want
> here is to stop the simulator when the target Unix system is halted,
> like:
> 
> $ /usr/local/qemu-mips/bin/qemu-system-mipsel -M pic32mx7-max32
> -nographic -monitor none -serial stdio -bios boot-max32.hex -kernel
> unix.hex -sd sdcard.img
> Board: chipKIT Max32
> Processor: M4K
> RAM size: 128 kbytes
> Load file: 'boot-max32.hex', 6720 bytes
> Load file: 'unix.hex', 144992 bytes
> Card0 image 'sdcard.img', 102888 kbytes
> [...]
> 2.11 BSD UNIX (pic32) (console)
> 
> login: root
> Password:
> Welcome to RetroBSD!
> erase, kill ^U, intr ^C
> # halt
> killing processes... done
> syncing disks... done
> halted
> $ _                           <-- QEMU terminated
> 
> On BSD, the halt command uses reboot(RB_HALT) system call to terminate
> the operating system. It essentially results in an endless loop on
> wait instruction with interrupts disabled., like "for(;;) {
> asm("wait"); }". For pic32 it makes little sense to continue
> simulation in this case.
> 
> Fortunately, I've found a solution which does not require modification
> of generic code. Everything can be done in the platform-specific part.

Why doesn't the OS do more than busy-loop in halt()? For example poke a
memory-mapped register, or make a firmware or semihosting call?

Chris

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