[PATCH] powerpc/asm: Mark cr0 as clobbered in mftb()

[Michael Ellerman]

Segher Boessenkool <segher at kernel.crashing.org> writes:

> Hi!
>
> On Mon, Jul 10, 2017 at 09:50:06PM +1000, Michael Ellerman wrote:
>> Oliver O'Halloran <oohall at gmail.com> writes:
>> 
>> > The workaround for the CELL timebase bug does not correctly mark cr0 as
>> > being clobbered. This can result in GCC making some poor^W completely
>> > broken optimisations.
>> 
>> Fruit 'n oats, how did we never notice that? Luck I guess. Or subtle
>> breakage that no one could pin down :E
>
> GCC does not use cr0 before it has used cr7, cr5, cr6, cr1 (unless some
> instruction _requires_ cr0, like record form ("dot") instructions, which
> until recently were disabled when targetting Cell), so it isn't too easy
> to hit the problem here.  Maybe Oliver used a very new GCC?  Or he was
> unlucky.

Aha. I think he actually hit the bug elsewhere and then noticed it here
also? And yeah I think it was a dot instruction that triggered it.

>> I'll tag it for stable.
>> 
>> Your change log is not entirely fair, it's not GCC's fault, we changed
>> register state without telling it, so it's on us :)  I'll reword it a
>> bit here.
>
> Yep, GCC works fine here, GIGO etc.  It isn't feasible to make GCC warn
> for most inline asm problems, either (we do not parse the mnemonics in
> the asm; this is a fundamental part of the design; you must express all
> constraints as, well, constraints).

Ack. It's a terrible terrible interface, but that's the price we pay for
wanting to insert arbitrary bits of asm inside an otherwise compiled
language :)

When I get the time I might go through our inline asm and convert all
the cmpwi x,z to cmpwi cr0,x,z, so at least the cr0 usage is a little
more obvious.

cheers