RE: [PATCH] Hexagon: lldb read/write predicate registers p0/p1/p2/p3

[<ltaylorsimpson@gmail.com>]

> -----Original Message-----
> From: Matheus Tavares Bernardino <quic_mathbern@quicinc.com>
> Sent: Wednesday, June 12, 2024 12:30 PM
> To: ltaylorsimpson@gmail.com
> Cc: qemu-devel@nongnu.org; bcain@quicinc.com; tedwood@quicinc.com;
> alex.bennee@linaro.org; quic_mathbern@quicinc.com;
> sidneym@quicinc.com; quic_mliebel@quicinc.com;
> richard.henderson@linaro.org; philmd@linaro.org; ale@rev.ng; anjo@rev.ng
> Subject: Re: [PATCH] Hexagon: lldb read/write predicate registers
> p0/p1/p2/p3
> 
> On Wed, 12 Jun 2024 10:42:39 -0600 Taylor Simpson
> <ltaylorsimpson@gmail.com> wrote:
> >
> > diff --git a/target/hexagon/gdbstub.c b/target/hexagon/gdbstub.c index
> > 502c6987f0..e67e627fc9 100644
> > --- a/target/hexagon/gdbstub.c
> > +++ b/target/hexagon/gdbstub.c
> > @@ -56,6 +64,15 @@ int hexagon_gdb_write_register(CPUState *cs,
> uint8_t *mem_buf, int n)
> >          return sizeof(target_ulong);
> >      }
> >
> > +    n -= TOTAL_PER_THREAD_REGS;
> > +
> > +    if (n < NUM_PREGS) {
> > +        env->pred[n] = ldtul_p(mem_buf);
> > +        return sizeof(uint8_t);
> 
> I wonder, shouldn't this be sizeof(target_ulong) since we wrote a
> target_ulong?

Good question.

From the architecture point of view, predicates are 8 bits (Section 2.2.5 of
the v73 Hexagon PRM).  However, we model them in QEMU as target_ulong
because TCG variables must be either 32 bits or 64 bits.  There isn't an
option for 8 bits.  Whenever we write to a predicate, do "and" with 0xff
first to ensure there are only 8 bits written (see gen_log_pred_write in
target/hexagon/genptr.c).

I did some more digging and here is what I found:
- Since we have bitsize="8" in hexagon-core.xml, lldb will reject any
attempt to write something larger.
  (lldb) reg write p1 0x1ff
  error: Failed to write register 'p1' with value '0x1ff': value 0x1ff is
too large to fit in a 1 byte unsigned integer value
- For the lldb "reg write" command, the return value from
hexagon_gdb_write_register isn't used.
- The only place the return value is used is in handle_write_all_regs.  This
function is called in response to a "G" packet from the debugger.  I don't
know if/when lldb uses this packet, but it seems like it would count on it
being 8 bits since that's what is in hexagon-core.xml.

Ted <tedwood@quicinc.com>, when would lldb generate a "G" packet, and what
assumptions will it make about the size of predicate registers?

Thanks,
Taylor