From 1c1e7e3c23dd25f938302428eeb22c3dda2c3427 Mon Sep 17 00:00:00 2001 From: Sebastian Andrzej Siewior Date: Mon, 28 Mar 2022 16:58:08 +0200 Subject: [PATCH] x86/percpu: Remove volatile from arch_raw_cpu_ptr(). The volatile attribute in the inline assembly of arch_raw_cpu_ptr() forces the compiler to always generate the code, even if the compiler can decide upfront that its result is not needed. For instance invoking __intel_pmu_disable_all(false) (like intel_pmu_snapshot_arch_branch_stack() does) leads to loading the address of &cpu_hw_events into the register while compiler knows that it has no need for it. This ends up with code like: | movq $cpu_hw_events, %rax #, tcp_ptr__ | add %gs:this_cpu_off(%rip), %rax # this_cpu_off, tcp_ptr__ | xorl %eax, %eax # tmp93 It also creates additional code within local_lock() with !RT && !LOCKDEP which is not desired. By removing the volatile attribute the compiler can place the function freely and avoid it if it is not needed in the end. By using the function twice the compiler properly caches only the variable offset and always loads the CPU-offset. this_cpu_ptr() also remains properly placed within a preempt_disable() sections because - arch_raw_cpu_ptr() assembly has a memory input ("m" (this_cpu_off)) - prempt_{dis,en}able() fundamentally has a 'barrier()' in it Therefore this_cpu_ptr() is already properly serialized and does not rely on the 'volatile' attribute. Remove volatile from arch_raw_cpu_ptr(). [ bigeasy: Added Linus' explanation why this_cpu_ptr() is not moved out of a preempt_disable() section without the 'volatile' attribute. ] Suggested-by: Linus Torvalds Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Peter Zijlstra (Intel) Link: https://lore.kernel.org/r/20220328145810.86783-2-bigeasy@linutronix.de --- arch/x86/include/asm/percpu.h | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h index a3c33b79fb86..13c0d63ed55e 100644 --- a/arch/x86/include/asm/percpu.h +++ b/arch/x86/include/asm/percpu.h @@ -38,9 +38,9 @@ #define arch_raw_cpu_ptr(ptr) \ ({ \ unsigned long tcp_ptr__; \ - asm volatile("add " __percpu_arg(1) ", %0" \ - : "=r" (tcp_ptr__) \ - : "m" (this_cpu_off), "0" (ptr)); \ + asm ("add " __percpu_arg(1) ", %0" \ + : "=r" (tcp_ptr__) \ + : "m" (this_cpu_off), "0" (ptr)); \ (typeof(*(ptr)) __kernel __force *)tcp_ptr__; \ }) #else