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x86/process/64: Use FSBSBASE in switch_to() if available
With the new FSGSBASE instructions, FS and GSABSE can be efficiently read and writen in __switch_to(). Use that capability to preserve the full state. This will enable user code to do whatever it wants with the new instructions without any kernel-induced gotchas. (There can still be architectural gotchas: movl %gs,%eax; movl %eax,%gs may change GSBASE if WRGSBASE was used, but users are expected to read the CPU manual before doing things like that.) This is a considerable speedup. It seems to save about 100 cycles per context switch compared to the baseline 4.6-rc1 behavior on a Skylake laptop. This is mostly due to avoiding the WRMSR operation. [ chang: 5~10% performance improvements were seen with a context switch benchmark that ran threads with different FS/GSBASE values (to the baseline 4.16). Minor edit on the changelog. ] [ tglx: Masaage changelog ] Signed-off-by: Andy Lutomirski <luto@kernel.org> Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Sasha Levin <sashal@kernel.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Andi Kleen <ak@linux.intel.com> Link: https://lkml.kernel.org/r/1557309753-24073-8-git-send-email-chang.seok.bae@intel.com Link: https://lkml.kernel.org/r/20200528201402.1708239-6-sashal@kernel.org
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@ -236,8 +236,18 @@ static __always_inline void save_fsgs(struct task_struct *task)
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{
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savesegment(fs, task->thread.fsindex);
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savesegment(gs, task->thread.gsindex);
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save_base_legacy(task, task->thread.fsindex, FS);
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save_base_legacy(task, task->thread.gsindex, GS);
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if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
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/*
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* If FSGSBASE is enabled, we can't make any useful guesses
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* about the base, and user code expects us to save the current
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* value. Fortunately, reading the base directly is efficient.
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*/
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task->thread.fsbase = rdfsbase();
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task->thread.gsbase = __rdgsbase_inactive();
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} else {
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save_base_legacy(task, task->thread.fsindex, FS);
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save_base_legacy(task, task->thread.gsindex, GS);
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}
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}
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/*
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@ -319,10 +329,22 @@ static __always_inline void load_seg_legacy(unsigned short prev_index,
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static __always_inline void x86_fsgsbase_load(struct thread_struct *prev,
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struct thread_struct *next)
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{
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load_seg_legacy(prev->fsindex, prev->fsbase,
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next->fsindex, next->fsbase, FS);
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load_seg_legacy(prev->gsindex, prev->gsbase,
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next->gsindex, next->gsbase, GS);
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if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
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/* Update the FS and GS selectors if they could have changed. */
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if (unlikely(prev->fsindex || next->fsindex))
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loadseg(FS, next->fsindex);
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if (unlikely(prev->gsindex || next->gsindex))
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loadseg(GS, next->gsindex);
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/* Update the bases. */
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wrfsbase(next->fsbase);
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__wrgsbase_inactive(next->gsbase);
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} else {
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load_seg_legacy(prev->fsindex, prev->fsbase,
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next->fsindex, next->fsbase, FS);
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load_seg_legacy(prev->gsindex, prev->gsbase,
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next->gsindex, next->gsbase, GS);
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}
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}
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static unsigned long x86_fsgsbase_read_task(struct task_struct *task,
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