Activating the sigma0 thread in the Fiasco kernel

[Adam Lackorzynski]

On Tue Mar 06, 2018 at 01:14:25 +0100, Paul Boddie wrote:
> On Tuesday 6. March 2018 00.46.29 Adam Lackorzynski wrote:
> > 
> > All what you write sounds good. In any case the eret must restore state
> > including setting the right interrupt state. Are you getting timer
> > interrupts when sigma0 shall run, or is there silence? Is ESC working to
> > get into jdb?
> 
> Thanks for the reply as usual! :-)
> 
> After Proc::cli is called in user_invoke, I don't think any interrupts will be 
> delivered, and if I display the status register, the IE (interrupt enable) bit 
> is indeed not set. So I wouldn't expect any timer interrupts unless something 
> else enables interrupts again, but I can't find any statement where this gets 
> done.
> 
> Here, I think that I *might* have transcribed some operation incorrectly, 
> leaving interrupts disabled when they should be re-enabled. The eret shouldn't 
> itself re-enable interrupts, as far as I remember from messing around with my 
> own boot payloads, since it merely clears the EXL (exception level) bit which 
> prevents interrupts if set (and then jumps to EPC, of course).
> 
> (Thinking about it, EXL isn't even set when I check the status register, but 
> if allowing interrupts in kernel mode, it is customary to clear it, from what 
> I have read, so maybe Fiasco does that.)
> 
> Now, I have transcribed the di instruction to the supposedly-equivalent status 
> register operations that clear IE, and the ei instruction to the operations 
> that set IE, both of these featuring in the Proc::cli and Proc::sti methods. 
> Maybe these instructions should be transcribed to set and clear EXL, however, 
> even though that is not what di and ei do.
> 
> As for jdb and UART interactions, I've had to use more primitive techniques 
> because I can't establish a reliable physical connection to the relevant pins. 
> Fortunately, I can take over the framebuffer and display simple bit patterns 
> (to keep debugging code at a minimum), and this is how I can comment on things 
> like the status register. Yes, it is a slow and tedious way of working, but 
> I've used it successfully before. :-)
> 
> Do you have any idea where this missing re-enabling statement might be, or 
> should I really be manipulating EXL instead of IE?

The asm code sets cp0_status upon exit which includes enabling
interrupts. Are you sure you're not getting any timer interrupts when
supposedly running inside sigma0? (Flipping some pixels in the timer
handler...)



Adam