linux-kernel - Re: [PATCH] compiler-gcc: disable -ftracer for _

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Date:   Tue, 8 May 2018 11:12:30 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>, lkml@...nx.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Marek <mmarek@...e.cz>, stable@...r.kernel.org
Subject: Re: [PATCH] compiler-gcc: disable -ftracer for __noclone functions

Paolo Bonzini <pbonzini@...hat.com> wrote:

> On 05/05/2018 19:22, Nadav Amit wrote:
>> Paolo Bonzini <pbonzini@...hat.com> wrote:
>> 
>>> -ftracer can duplicate asm blocks causing compilation to fail in
>>> noclone functions.  For example, KVM declares a global variable
>>> in an asm like
>>> 
>>>   asm("2: ... \n
>>>        .pushsection data \n
>>>        .global vmx_return \n
>>>        vmx_return: .long 2b");
>>> 
>>> and -ftracer causes a double declaration.
>>> 
>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>> Cc: Michal Marek <mmarek@...e.cz>
>>> Cc: stable@...r.kernel.org
>>> Cc: kvm@...r.kernel.org
>>> Reported-by: Linda Walsh <lkml@...nx.org>
>>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>>> ---
>>> include/linux/compiler-gcc.h | 2 +-
>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>> 
>>> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
>>> index 22ab246feed3..eeae401a2412 100644
>>> --- a/include/linux/compiler-gcc.h
>>> +++ b/include/linux/compiler-gcc.h
>>> @@ -199,7 +199,7 @@
>>> #define unreachable() __builtin_unreachable()
>>> 
>>> /* Mark a function definition as prohibited from being cloned. */
>>> -#define __noclone	__attribute__((__noclone__))
>>> +#define __noclone	__attribute__((__noclone__, __optimize__("no-tracer")))
>> 
>> [ Bringing the thread back from the dead for context ]
>> 
>> Setting different optimization attributes to certain functions apparently
>> prevents gcc from inlining functions with different “optimizations”. This
>> results in poor compilation - most notably of vmx_vcpu_run() - and causes
>> short functions such as to_vmx() not to be inlined.
> 
> Hmm, I suppose we need to disable -fno-tracer globally...  Has this
> changed in recent versions of GCC?

I don’t think so. I played with optimization attributes of gcc some time ago
and saw the same behavior. Unfortunately, I don’t easily find the place in
gcc that implements this behavior.