Re: [Qemu-devel] MIPS interrupt handling

[Thiemo Seufer <ths@networkno.de>]

Fabrice Bellard wrote:
> Thiemo Seufer wrote:
> >Fabrice Bellard wrote:
> >
> >>I just looked at the MIPS file target-mips/op_helper.c and I don't 
> >>understand why IRQs need to be handled in op_helper.c:do_mtc0() with reg 
> >>= 12.
> >
> >
> >Register 12 is the cp0_status register, it defines which interrupts are
> >masked/enabled/disabled. Btw, I have a patch which moves this to op.c,
> >this should improve performance a bit (and avoids the TB stop for
> >most mtc0 writes).
> 
> op.c should only contain small functions so it is not a good idea. TB 
> stop after mtc0 is needed at least when the TLB are modified or to 
> handle the interrupts.

Agreed for the cp0_status part, the other functions are small (probably
except cp_cause) and most of them don't need TB stop.

> The current handling of interrupts in mtc0 must 
> be suppressed ASAP as it is not useful and complicates the code.
> 
> >>IMHO, the corresponding code should be deleted because the TB is 
> >>forced to terminate after mtc0 so that the IRQs can be handled in the 
> >>main loop in cpu-exec.c.
> >>
> >>Moreover, clearing CPU_INTERRUPT_HARD in do_mtc0() is almost surely a bug 
> >>!
> >
> >
> >Somehow the interrupt assert has to be prevented when St0_IE is cleared.
> >That's probably also a job for the main loop, but there may be a race
> >condition (haven't looked yet). Empirically, it works well. :-)
> 
> If ST0_IE means interrupt enable, the interrupt assert must not be 
> suppressed, but I did not read yet this part of the MIPS spec...

As I tried to explain, I suspect there's a race between disabling
interrupts via ST0_IE and turning them off in the exception handling
code. For the enable part, it shouldn't matter that much.


Thiemo