queue-4.9/0040-x86-speculation-Avoid-__switch_to_xtra-calls.patch

   1 From ccfa6e5a4d38ff8c1c1bc7b07415ed1afd303f4e Mon Sep 17 00:00:00 2001
   2 From: Thomas Gleixner <tglx@linutronix.de>
   3 Date: Sun, 25 Nov 2018 19:33:48 +0100
   4 Subject: [PATCH 40/76] x86/speculation: Avoid __switch_to_xtra() calls
   5
   6 commit 5635d99953f04b550738f6f4c1c532667c3fd872 upstream.
   7
   8 The TIF_SPEC_IB bit does not need to be evaluated in the decision to invoke
   9 __switch_to_xtra() when:
  10
  11  - CONFIG_SMP is disabled
  12
  13  - The conditional STIPB mode is disabled
  14
  15 The TIF_SPEC_IB bit still controls IBPB in both cases so the TIF work mask
  16 checks might invoke __switch_to_xtra() for nothing if TIF_SPEC_IB is the
  17 only set bit in the work masks.
  18
  19 Optimize it out by masking the bit at compile time for CONFIG_SMP=n and at
  20 run time when the static key controlling the conditional STIBP mode is
  21 disabled.
  22
  23 Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
  24 Reviewed-by: Ingo Molnar <mingo@kernel.org>
  25 Cc: Peter Zijlstra <peterz@infradead.org>
  26 Cc: Andy Lutomirski <luto@kernel.org>
  27 Cc: Linus Torvalds <torvalds@linux-foundation.org>
  28 Cc: Jiri Kosina <jkosina@suse.cz>
  29 Cc: Tom Lendacky <thomas.lendacky@amd.com>
  30 Cc: Josh Poimboeuf <jpoimboe@redhat.com>
  31 Cc: Andrea Arcangeli <aarcange@redhat.com>
  32 Cc: David Woodhouse <dwmw@amazon.co.uk>
  33 Cc: Tim Chen <tim.c.chen@linux.intel.com>
  34 Cc: Andi Kleen <ak@linux.intel.com>
  35 Cc: Dave Hansen <dave.hansen@intel.com>
  36 Cc: Casey Schaufler <casey.schaufler@intel.com>
  37 Cc: Asit Mallick <asit.k.mallick@intel.com>
  38 Cc: Arjan van de Ven <arjan@linux.intel.com>
  39 Cc: Jon Masters <jcm@redhat.com>
  40 Cc: Waiman Long <longman9394@gmail.com>
  41 Cc: Greg KH <gregkh@linuxfoundation.org>
  42 Cc: Dave Stewart <david.c.stewart@intel.com>
  43 Cc: Kees Cook <keescook@chromium.org>
  44 Link: https://lkml.kernel.org/r/20181125185005.374062201@linutronix.de
  45 [bwh: Backported to 4.9: adjust context]
  46 Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
  47 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
  48 ---
  49  arch/x86/include/asm/thread_info.h | 13 +++++++++++--
  50  arch/x86/kernel/process.h          | 15 +++++++++++++++
  51  2 files changed, 26 insertions(+), 2 deletions(-)
  52
  53 diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
  54 index 389b95bb1456..cd3f6be025be 100644
  55 --- a/arch/x86/include/asm/thread_info.h
  56 +++ b/arch/x86/include/asm/thread_info.h
  57 @@ -142,9 +142,18 @@ struct thread_info {
  58         _TIF_NOHZ)
  59
  60  /* flags to check in __switch_to() */
  61 -#define _TIF_WORK_CTXSW                                                        \
  62 +#define _TIF_WORK_CTXSW_BASE                                           \
  63         (_TIF_IO_BITMAP|_TIF_NOTSC|_TIF_BLOCKSTEP|                      \
  64 -        _TIF_SSBD|_TIF_SPEC_IB)
  65 +        _TIF_SSBD)
  66 +
  67 +/*
  68 + * Avoid calls to __switch_to_xtra() on UP as STIBP is not evaluated.
  69 + */
  70 +#ifdef CONFIG_SMP
  71 +# define _TIF_WORK_CTXSW       (_TIF_WORK_CTXSW_BASE | _TIF_SPEC_IB)
  72 +#else
  73 +# define _TIF_WORK_CTXSW       (_TIF_WORK_CTXSW_BASE)
  74 +#endif
  75
  76  #define _TIF_WORK_CTXSW_PREV (_TIF_WORK_CTXSW|_TIF_USER_RETURN_NOTIFY)
  77  #define _TIF_WORK_CTXSW_NEXT (_TIF_WORK_CTXSW)
  78 diff --git a/arch/x86/kernel/process.h b/arch/x86/kernel/process.h
  79 index 020fbfac3a27..898e97cf6629 100644
  80 --- a/arch/x86/kernel/process.h
  81 +++ b/arch/x86/kernel/process.h
  82 @@ -2,6 +2,8 @@
  83  //
  84  // Code shared between 32 and 64 bit
  85
  86 +#include <asm/spec-ctrl.h>
  87 +
  88  void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p);
  89
  90  /*
  91 @@ -14,6 +16,19 @@ static inline void switch_to_extra(struct task_struct *prev,
  92         unsigned long next_tif = task_thread_info(next)->flags;
  93         unsigned long prev_tif = task_thread_info(prev)->flags;
  94
  95 +       if (IS_ENABLED(CONFIG_SMP)) {
  96 +               /*
  97 +                * Avoid __switch_to_xtra() invocation when conditional
  98 +                * STIPB is disabled and the only different bit is
  99 +                * TIF_SPEC_IB. For CONFIG_SMP=n TIF_SPEC_IB is not
 100 +                * in the TIF_WORK_CTXSW masks.
 101 +                */
 102 +               if (!static_branch_likely(&switch_to_cond_stibp)) {
 103 +                       prev_tif &= ~_TIF_SPEC_IB;
 104 +                       next_tif &= ~_TIF_SPEC_IB;
 105 +               }
 106 +       }
 107 +
 108         /*
 109          * __switch_to_xtra() handles debug registers, i/o bitmaps,
 110          * speculation mitigations etc.
 111 --
 112 2.21.0
 113