Re: [Qemu-devel] Wrong reset of MIPS hflags EXL after interrupt?

[Dirk Behme]

Thiemo Seufer wrote:
> Dirk Behme wrote:
> > I'm not sure, but while playing with MIPS interrupts, it 
> > seems to me that something with reset of interrupt flag 
> > MIPS_HFLAG_EXL (0x04) at exception exit (eret) is wrong. It 
> > seems to me that only one interrupt is executed because 
> > after eret, MIPS_HFLAG_EXL stays set in env->hflags. Then, 
> > at next interrupt, system correctly checks for 
> > MIPS_HFLAG_EXL, but this is still set and no further 
> > interrupt happens.
>
> This explains some weirdness I saw on my hacked up qemu
> when running a mips32r2-compiled Linux kernel.

This was the reason why I first tried it with a small test 
program before using Linux ;)

> > Debugging shows that op_eret() in MIPS op.c correctly reset 
> > this bit: env->hflags &= ~MIPS_HFLAG_EXL; But debug output 
> > at end of e.g. save_cpu_state() (debug output of ctx->hflags 
> > and ctx->saved_hflags ) or in function which tries to issue 
> > (next) timer interrupt (debug output of env->hflags) 
> > MIPS_HFLAG_EXL is still (again?) set everywhere. Looks like 
> > the correct env->hflags from op_eret() is overwritten 
> > somewhere later with wrong value.
> >
> > These three ctx->hflags, ctx->saved_hflags and env->hflags 
> > are confusing me ;) Where are they synchronized after eret? 
> > Or who overwrites the env->hflags correctly set by eret 
> > again? Any ideas, why eret sets env->hflags correctly and 
> > later global env->hflags has still/again wrong value? Any 
> > other hints?
>
> AFAIU qemu maintains an environment stack, I guess popping the
> environment restores the old flag contents.

Anybody with a short explanation of the basics of this? I 
think this would really help debugging this issue.

Many thanks

Dirk