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Subject: Re: [PATCH] compiler-gcc: disable -ftracer for __noclone functions
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <86f2fb42-b0eb-46d2-0aed-7efdb306bfe0@redhat.com>
Date:   Tue, 8 May 2018 11:12:30 -0700
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        kvm list <kvm@vger.kernel.org>, lkml@tlinx.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Marek <mmarek@suse.cz>, stable@vger.kernel.org
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References: <1459410252-15531-1-git-send-email-pbonzini@redhat.com>
 <6BAD64FE-CC59-40C3-9C42-90BB8EEE5453@gmail.com>
 <86f2fb42-b0eb-46d2-0aed-7efdb306bfe0@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
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Paolo Bonzini <pbonzini@redhat.com> wrote:

> On 05/05/2018 19:22, Nadav Amit wrote:
>> Paolo Bonzini <pbonzini@redhat.com> wrote:
>>=20
>>> -ftracer can duplicate asm blocks causing compilation to fail in
>>> noclone functions.  For example, KVM declares a global variable
>>> in an asm like
>>>=20
>>>   asm("2: ... \n
>>>        .pushsection data \n
>>>        .global vmx_return \n
>>>        vmx_return: .long 2b");
>>>=20
>>> and -ftracer causes a double declaration.
>>>=20
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: Michal Marek <mmarek@suse.cz>
>>> Cc: stable@vger.kernel.org
>>> Cc: kvm@vger.kernel.org
>>> Reported-by: Linda Walsh <lkml@tlinx.org>
>>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>>> ---
>>> include/linux/compiler-gcc.h | 2 +-
>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>=20
>>> 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()
>>>=20
>>> /* Mark a function definition as prohibited from being cloned. */
>>> -#define __noclone	__attribute__((__noclone__))
>>> +#define __noclone	__attribute__((__noclone__, =
__optimize__("no-tracer")))
>>=20
>> [ Bringing the thread back from the dead for context ]
>>=20
>> Setting different optimization attributes to certain functions =
apparently
>> prevents gcc from inlining functions with different =
=E2=80=9Coptimizations=E2=80=9D. This
>> results in poor compilation - most notably of vmx_vcpu_run() - and =
causes
>> short functions such as to_vmx() not to be inlined.
>=20
> Hmm, I suppose we need to disable -fno-tracer globally...  Has this
> changed in recent versions of GCC?

I don=E2=80=99t think so. I played with optimization attributes of gcc =
some time ago
and saw the same behavior. Unfortunately, I don=E2=80=99t easily find =
the place in
gcc that implements this behavior.