Re: [PATCH v4.2] powerpc/modules: Don't try to restore r2 after a sibling call

[Naveen N. Rao]

Kamalesh Babulal wrote:
> On Thursday 16 November 2017 11:15 PM, Josh Poimboeuf wrote:
> > On Thu, Nov 16, 2017 at 06:39:03PM +0530, Naveen N. Rao wrote:
> > > Josh Poimboeuf wrote:
> > > > On Wed, Nov 15, 2017 at 02:58:33PM +0530, Naveen N. Rao wrote:
> > > > > > +int instr_is_link_branch(unsigned int instr)
> > > > > > +{
> > > > > > +	return (instr_is_branch_iform(instr) || instr_is_branch_bform(instr)) &&
> > > > > > +	       (instr & BRANCH_SET_LINK);
> > > > > > +}
> > > > > > +
> > > > >
> > > > > Nitpicking here, but since we're not considering the other branch forms,
> > > > > perhaps this can be renamed to instr_is_link_relative_branch() (or maybe
> > > > > instr_is_relative_branch_link()), just so we're clear :)
> > > >
> > > > My understanding is that the absolute/relative bit isn't a "form", but
> > > > rather a bit that can be set for either the b-form (conditional) or the
> > > > i-form (unconditional).  And the above function isn't checking the
> > > > absolute bit, so it isn't necessarily a relative branch.  Or did I miss
> > > > something?
> > >
> > > Ah, good point. I was coming from the fact that we are only considering the
> > > i-form and b-form branches and not the lr/ctr/tar based branches, which are
> > > always absolute branches, but can also set the link register.
> >
> > Hm, RISC is more complicated than I realized ;-)

As long as 'RISC' gets people to take a look ;D

> > > Thinking about this more, aren't we only interested in relative branches
> > > here (for relocations), so can we actually filter out the absolute branches?
> > > Something like this?
> > >
> > > int instr_is_relative_branch_link(unsigned int instr)
> > > {
> > > 	return ((instr_is_branch_iform(instr) || instr_is_branch_bform(instr)) &&
> > > 	       !(instr & BRANCH_ABSOLUTE) && (instr & BRANCH_SET_LINK));
> >
> > Yeah, makes sense to me.  Here's another try (also untested).  If this
> > looks ok, Kamalesh would you mind testing again?

Thanks. That looks good to me.
Acked-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>

> > ----8<----
> >
> > From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > Subject: [PATCH v4.2] powerpc/modules: Don't try to restore r2 after a sibling call
> >
> > When attempting to load a livepatch module, I got the following error:
> >
> >   module_64: patch_module: Expect noop after relocate, got 3c820000
> >
> > The error was triggered by the following code in
> > unregister_netdevice_queue():
> >
> >   14c:   00 00 00 48     b       14c <unregister_netdevice_queue+0x14c>
> >                          14c: R_PPC64_REL24      net_set_todo
> >   150:   00 00 82 3c     addis   r4,r2,0
> >
> > GCC didn't insert a nop after the branch to net_set_todo() because it's
> > a sibling call, so it never returns.  The nop isn't needed after the
> > branch in that case.
> >
> > Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
>
> Reviewed-and-tested-by: Kamalesh Babulal <kamalesh@xxxxxxxxxxxxxxxxxx>

Thanks, Kamalesh!


- Naveen