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

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Message-Id: <1510992250.emxjvilqnz.naveen@linux.ibm.com>
Date:   Sat, 18 Nov 2017 14:03:36 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
Cc:     Balbir Singh <bsingharora@...il.com>, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, live-patching@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH v4.2] powerpc/modules: Don't try to restore r2 after a
 sibling call

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@...ux.vnet.ibm.com>

>>
>> ----8<----
>>
>> From: Josh Poimboeuf <jpoimboe@...hat.com>
>> 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@...hat.com>
> 
> Reviewed-and-tested-by: Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>

Thanks, Kamalesh!


- Naveen