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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Eric Dumazet <edumazet@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Eric Dumazet <eric.dumazet@gmail.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	netdev@vger.kernel.org, Marco Elver <elver@google.com>,
	Uros Bizjak <ubizjak@gmail.com>
Subject: Re: [PATCH] locking/local_lock: Reduce local_[un]lock_nested_bh()
 overhead
Message-ID: <20260311155539.3Aikrlaz@linutronix.de>
References: <20260309122055.1226507-1-edumazet@google.com>
 <20260309134355.GC606826@noisy.programming.kicks-ass.net>
 <CANn89iJTw1Wu8jOGuvGKnWixo86LSRXNzgKkj2A=csrYvj9KfQ@mail.gmail.com>
 <CANn89i+7Pqf9pdZtic3kesKakv9+5TOgfWSce=M065wNmaBHsg@mail.gmail.com>
 <CANn89iKyPZUqmirYza5ewD2dBQfZCMunG771x7cMpUFkvw9x9w@mail.gmail.com>
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In-Reply-To: <CANn89iKyPZUqmirYza5ewD2dBQfZCMunG771x7cMpUFkvw9x9w@mail.gmail.com>

On 2026-03-09 15:52:34 [+0100], Eric Dumazet wrote:
> > +void ericeric(void);
> > +void ericeric(void)
> > +{
> > +       raw_cpu_read_long(this_cpu_off);
> > +       raw_cpu_read_long(this_cpu_off);
> > +}
> >
> > I am guessing __raw_cpu_read() is forcing the asm ?
> 
> Might be a clang issue. Oh well.

So the difference is that with gcc we have USE_X86_SEG_SUPPORT and with
llvm we don't. This leads to two asm statements with LLVM of which only
one is eliminated. This optimisation origins in commit ca4256348660c
("x86/percpu: Use C for percpu read/write accessors").

__seg_fs and __seg_gs is supported by LLVM but enabling it leads to tons
warnings and aborts later.

Is there something missing in LLVM? The generated code for
raw_cpu_read_long(this_cpu_off) looks fine.

Sebastian