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x86/process/64: Use FSBSBASE in switch_to() if available

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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
This commit is contained in:
Andy Lutomirski 2020-05-28 16:13:51 -04:00 committed by Thomas Gleixner
parent 6758034e4d
commit 673903495c
1 changed files with 28 additions and 6 deletions
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34
arch/x86/kernel/process_64.c
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@ -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,