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

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Message-ID: <86f2fb42-b0eb-46d2-0aed-7efdb306bfe0@redhat.com>
Date:   Tue, 8 May 2018 19:58:25 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Nadav Amit <nadav.amit@...il.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

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?

Paolo