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

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index ef2f755..8ccc587 100644 (file)
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -236,8 +236,18 @@ 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);
+       }
 }
 
 /*
@@ -319,10 +329,22 @@ 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,