[libre-riscv-dev] [Bug 325] create POWER9 TRAP pipeline

[bugzilla-daemon@libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=325

--- Comment #91 from Luke Kenneth Casson Leighton <lkcl@lkcl.net> ---
(In reply to Cole Poirier from comment #90)
> (In reply to Luke Kenneth Casson Leighton from comment #89)
> > ok so i added two new signals to Decode2Execute1Type, called
> > traptype and trapaddr.  the idea is that rather than get main_stage.py
> > to *hard-code* what type of trap it is, and *hard-code* the address to
> > jump to, the *decoder* says what the trap type is, and where to jump to.
> > 
> > in this way we can detect interrupts, privileged instructions, and so on,
> > in the *decoder*.
> > 
> > unfortunately... this means that the decoder needs access to MSR.  whoops.
> > this is slightly unfortunate, because up until now, PowerDecoder2 needed
> > absolutely nothing - no information *at all* - about the "state".  didn't
> > need the PC, didn't need anything.
> > 
> > 
> > +        with m.If(op.internal_op == InternalOp.OP_TRAP):
> > +            comb += e.traptype.eq(TT_TRAP) # request trap interrupt
> > +            comb += e.trapaddr.eq(0x70)    # addr=0x700 (strip first nibble)
> > +
> > +        return m
> >  
> > +        # privileged instruction
> > +        with m.If(instr_is_privileged(m, op.internal_op, e.insn) &
> > +                  msr[MSR_PR]):
> > +            # privileged instruction trap
> > +            comb += op.internal_op.eq(InternalOp.OP_TRAP)
> > +            comb += e.traptype.eq(TT_PRIV) # request privileged instruction
> > +            comb += e.trapaddr.eq(0x70)    # addr=0x700 (strip first nibble)
> 
> Very cool. Can you comment on the implications of the decoder needing access
> to MSR here?

sigh it's annoying. however given that we gave to pass state information
around, containing the PC, adding the MSR to that,state is not so bad.  it's
just that it messes with dependency tracking.

> or is that meant to be a comment on a different bug report. If
> so, please link it here, because I'd like to understand the decoder better.

the decoder's really not difficult.  funnily enough i updated this page

https://libre-soc.org/Documentation/SOC/index/

it links to PowerDecoder.  the thing yo remember is that there is no hardcoded
list of junk.  i will edit the page further, or we discuss on list. thus
bugrrport us intended for TRAP

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