RE: [v3 05/12] x86/fsgsbase/64: Preserve FS/GS state in __switch_to() if FSGSBASE is on

["Bae, Chang Seok" <chang.seok.bae@intel.com>]

On Tue, Oct 24, 2018 at 12:21 PM Andy Lutomirski <luto@kernel.org> wrote:
> On Tue, Oct 23, 2018 at 11:43 AM Chang S. Bae <chang.seok.bae@intel.com>
> wrote:
> >
> > 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. 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>
> > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > ---
> >  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 fcf18046c3d6..1d975cadc256 100644
> > --- a/arch/x86/kernel/process_64.c
> > +++ b/arch/x86/kernel/process_64.c
> > @@ -238,8 +238,18 @@ 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 = rd_inactive_gsbase();
> > +       } else {
> > +               save_base_legacy(task, task->thread.fsindex, FS);
> > +               save_base_legacy(task, task->thread.gsindex, GS);
> > +       }
> >  }
> >
> >  #if IS_ENABLED(CONFIG_KVM)
> > @@ -318,10 +328,22 @@ 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);
> > +               wr_inactive_gsbase(next->gsbase);
> 
> Aha, I see what you're doing with the FSGSBASE-optimized version being
> out of line.  But it's way too unclear from the code.  You should name
> the helper wrgsbase_inactive or maybe __wrgsbase_inactive() to
> emphasize that you're literally using the WRGSBASE instruction.  (Or
> it's Xen PV equivalent.  Hmm.)

Okay. Will rename the relevants.