Messages in this thread Patch in this message | | | From | "Chang S. Bae" <> | Subject | [RESEND PATCH v6 05/12] x86/fsgsbase/64: Preserve FS/GS state in __switch_to() if FSGSBASE is on | Date | Fri, 15 Mar 2019 13:06:38 -0700 |
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From: Andy Lutomirski <luto@kernel.org>
With the new FSGSBASE instructions, we can efficiently read and write the FSBASE and GSBASE 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 my Skylake laptop.
[ chang: 5~10% performance improvements were seen by a context switch benchmark that ran threads with different FS/GSBASE values (to the baseline 4.16). Minor edit on the changelog. ]
Signed-off-by: Andy Lutomirski <luto@kernel.org> Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com> Reviewed-by: Andi Kleen <ak@linux.intel.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@kernel.org> --- arch/x86/kernel/process_64.c | 34 ++++++++++++++++++++++++++++------ 1 file changed, 28 insertions(+), 6 deletions(-)
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c index 87b5ffce2a47..d3abe4748d8b 100644 --- a/arch/x86/kernel/process_64.c +++ b/arch/x86/kernel/process_64.c @@ -245,8 +245,18 @@ static __always_inline void save_fsgs(struct task_struct *task) { savesegment(fs, task->thread.fsindex); savesegment(gs, task->thread.gsindex); - save_base_legacy(task, task->thread.fsindex, FS); - save_base_legacy(task, task->thread.gsindex, GS); + if (static_cpu_has(X86_FEATURE_FSGSBASE)) { + /* + * If FSGSBASE is enabled, we can't make any useful guesses + * about the base, and user code expects us to save the current + * value. Fortunately, reading the base directly is efficient. + */ + task->thread.fsbase = rdfsbase(); + task->thread.gsbase = __rdgsbase_inactive(); + } else { + save_base_legacy(task, task->thread.fsindex, FS); + save_base_legacy(task, task->thread.gsindex, GS); + } } #if IS_ENABLED(CONFIG_KVM) @@ -325,10 +335,22 @@ static __always_inline void load_seg_legacy(unsigned short prev_index, static __always_inline void x86_fsgsbase_load(struct thread_struct *prev, struct thread_struct *next) { - load_seg_legacy(prev->fsindex, prev->fsbase, - next->fsindex, next->fsbase, FS); - load_seg_legacy(prev->gsindex, prev->gsbase, - next->gsindex, next->gsbase, GS); + if (static_cpu_has(X86_FEATURE_FSGSBASE)) { + /* Update the FS and GS selectors if they could have changed. */ + if (unlikely(prev->fsindex || next->fsindex)) + loadseg(FS, next->fsindex); + if (unlikely(prev->gsindex || next->gsindex)) + loadseg(GS, next->gsindex); + + /* Update the bases. */ + wrfsbase(next->fsbase); + __wrgsbase_inactive(next->gsbase); + } else { + load_seg_legacy(prev->fsindex, prev->fsbase, + next->fsindex, next->fsbase, FS); + load_seg_legacy(prev->gsindex, prev->gsbase, + next->gsindex, next->gsbase, GS); + } } static unsigned long x86_fsgsbase_read_task(struct task_struct *task, -- 2.19.1
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