Re: [PATCH 1/4] Cleanups to FreeBSD/mips native register operations.

[John Baldwin <jhb@freebsd.org>]

On Thursday, April 13, 2017 10:48:48 AM Luis Machado wrote:
> On 04/12/2017 01:37 PM, John Baldwin wrote:
> > Compare against the "raw" PC register number instead of the cooked
> > register number when determining if a register was handled by
> > PT_GETREGS.  Previously the register fetch/store operations only tried
> > PT_GETREGS to fetch any individual register.  The result was that
> > fetching or storing an individual register not covered by PT_GETREGS
> > (such as floating point registers) did not work.
> >
> > While here, remove an early exit to simplify the code flow from the
> > PT_GETREGS / PT_SETREGS case, and add a getfpregs_supplies similar to
> > getregs_supplies to describe the registers supplied by PT_GETFPREGS
> > and PT_SETFPREGS.
> >
> > gdb/ChangeLog:
> >
> > 	* mips-fbsd-nat.c (getregs_supplies): Fix upper bound comparison.
> > 	(getpfpregs_supplies): New function.
> > 	(mips_fbsd_fetch_inferior_registers): Remove early exit and use
> > 	getfpregs_supplies.
> > 	(mips_fbsd_store_inferior_registers): Likewise.
> > ---
> >  gdb/ChangeLog       |  8 ++++++++
> >  gdb/mips-fbsd-nat.c | 26 ++++++++++++++------------
> >  2 files changed, 22 insertions(+), 12 deletions(-)
> 
> Only a few nits.
> 
> > diff --git a/gdb/mips-fbsd-nat.c b/gdb/mips-fbsd-nat.c
> > index 078df52db6..e2ed63e829 100644
> > --- a/gdb/mips-fbsd-nat.c
> > +++ b/gdb/mips-fbsd-nat.c
> > @@ -37,7 +37,16 @@ static bool
> >  getregs_supplies (struct gdbarch *gdbarch, int regnum)
> >  {
> >    return (regnum >= MIPS_ZERO_REGNUM
> > -	  && regnum <= gdbarch_pc_regnum (gdbarch));
> > +	  && regnum <= mips_regnum (gdbarch)->pc);
> > +}
> 
> Can the BSD backend override the pc value in gdbarch_pc_regnum (...) so 
> it fits what is expected? Or is this a case where the cooked pc register 
> number is still useful and we need to handle things differently for the 
> raw pc register number?

The cooked value is too large.  In particular, the existing range
right now goes from "0" to "cooked PC" which includes
"raw GP + raw FP + cooked GP".  This means that a request for a raw
FP register will see 'getregs_supplies() return true and will try
to satisfy it via PT_GETREGS (which doesn't work).  The end result is that
the register just isn't found.

> > +
> > +/* Determine if PT_GETFPREGS fetches this register.  */
> 
> Pedantically, "... fetches REGNUM".
> 
> > @@ -47,9 +56,9 @@ static void
> >  mips_fbsd_fetch_inferior_registers (struct target_ops *ops,
> >  				    struct regcache *regcache, int regnum)
> >  {
> > +  struct gdbarch *gdbarch = get_regcache_arch (regcache);
> >    pid_t pid = get_ptrace_pid (regcache_get_ptid (regcache));
> >
> > -  struct gdbarch *gdbarch = get_regcache_arch (regcache);
> >    if (regnum == -1 || getregs_supplies (gdbarch, regnum))
> 
> With C++ we can leave the declaration closer to its use. Same in the 
> other case below.

Yes, though in this case, gdbarch is actually used sooner than 'pid'.  I
can revert these though.

> > @@ -58,12 +67,9 @@ mips_fbsd_fetch_inferior_registers (struct target_ops *ops,
> >  	perror_with_name (_("Couldn't get registers"));
> >
> >        mips_fbsd_supply_gregs (regcache, regnum, &regs, sizeof (register_t));
> > -      if (regnum != -1)
> > -	return;
> >      }
> >
> > -  if (regnum == -1
> > -      || regnum >= gdbarch_fp0_regnum (get_regcache_arch (regcache)))
> > +  if (regnum == -1 || getfpregs_supplies (gdbarch, regnum))
> 
> Does MIPS on fsbd handle vector registers? I ask this because regnum >= 
> "fp0 regnum" may mean anything other than general purpose registers.

It does not, but I'm working on a research CPU that is an extension to MIPS
(CHERI) and it uses a third bank of registers that live above 'fp' with
a separate ptrace interface, etc.  I don't know that the CHERI bits will ever
be upstreamed since it is a research design, but this newer check seems
strictly more correct as it only uses PT_GETFPREGS to fetch registers that
are actually supported (e.g. it won't try to use it to fetch fir which
FreeBSD doesn't export via PT_GETFPREGS).

> If there are vector (or higher-numbered registers), the new conditional 
> block means something different compared to the old one.

I think that the new conditional is more correct in the case of higher
numbered registers as it means we don't try to use PT_GETFPREGS to
fetch a register it doesn't support.

> If not, then the change looks sane.

-- 
John Baldwin