linux-kernel - [PATCH 3.16 51/86] x86/speculation: Avoid __switch_to

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Message-ID: <lsq.1558022133.122628530@decadent.org.uk>
Date:   Thu, 16 May 2019 16:55:33 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Andrea Arcangeli" <aarcange@...hat.com>,
        "Arjan van de Ven" <arjan@...ux.intel.com>,
        "Tom Lendacky" <thomas.lendacky@....com>,
        "Greg KH" <gregkh@...uxfoundation.org>,
        "Josh Poimboeuf" <jpoimboe@...hat.com>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Jon Masters" <jcm@...hat.com>,
        "Dave Stewart" <david.c.stewart@...el.com>,
        "Waiman Long" <longman9394@...il.com>,
        "Casey Schaufler" <casey.schaufler@...el.com>,
        "Andy Lutomirski" <luto@...nel.org>,
        "Tim Chen" <tim.c.chen@...ux.intel.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Kees Cook" <keescook@...omium.org>,
        "David Woodhouse" <dwmw@...zon.co.uk>,
        "Asit Mallick" <asit.k.mallick@...el.com>,
        "Jiri Kosina" <jkosina@...e.cz>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "Dave Hansen" <dave.hansen@...el.com>,
        "Andi Kleen" <ak@...ux.intel.com>, "Ingo Molnar" <mingo@...nel.org>
Subject: [PATCH 3.16 51/86] x86/speculation: Avoid __switch_to_xtra() calls

3.16.68-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Thomas Gleixner <tglx@...utronix.de>

commit 5635d99953f04b550738f6f4c1c532667c3fd872 upstream.

The TIF_SPEC_IB bit does not need to be evaluated in the decision to invoke
__switch_to_xtra() when:

 - CONFIG_SMP is disabled

 - The conditional STIPB mode is disabled

The TIF_SPEC_IB bit still controls IBPB in both cases so the TIF work mask
checks might invoke __switch_to_xtra() for nothing if TIF_SPEC_IB is the
only set bit in the work masks.

Optimize it out by masking the bit at compile time for CONFIG_SMP=n and at
run time when the static key controlling the conditional STIBP mode is
disabled.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Reviewed-by: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Jiri Kosina <jkosina@...e.cz>
Cc: Tom Lendacky <thomas.lendacky@....com>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: David Woodhouse <dwmw@...zon.co.uk>
Cc: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: Andi Kleen <ak@...ux.intel.com>
Cc: Dave Hansen <dave.hansen@...el.com>
Cc: Casey Schaufler <casey.schaufler@...el.com>
Cc: Asit Mallick <asit.k.mallick@...el.com>
Cc: Arjan van de Ven <arjan@...ux.intel.com>
Cc: Jon Masters <jcm@...hat.com>
Cc: Waiman Long <longman9394@...il.com>
Cc: Greg KH <gregkh@...uxfoundation.org>
Cc: Dave Stewart <david.c.stewart@...el.com>
Cc: Kees Cook <keescook@...omium.org>
Link: https://lkml.kernel.org/r/20181125185005.374062201@linutronix.de
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/x86/include/asm/thread_info.h | 13 +++++++++++--
 arch/x86/kernel/process.h          | 15 +++++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)

--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -149,9 +149,18 @@ struct thread_info {
 	 _TIF_USER_RETURN_NOTIFY | _TIF_UPROBE)
 
 /* flags to check in __switch_to() */
-#define _TIF_WORK_CTXSW							\
+#define _TIF_WORK_CTXSW_BASE						\
 	(_TIF_IO_BITMAP|_TIF_NOTSC|_TIF_BLOCKSTEP|			\
-	 _TIF_SSBD|_TIF_SPEC_IB)
+	 _TIF_SSBD)
+
+/*
+ * Avoid calls to __switch_to_xtra() on UP as STIBP is not evaluated.
+ */
+#ifdef CONFIG_SMP
+# define _TIF_WORK_CTXSW	(_TIF_WORK_CTXSW_BASE | _TIF_SPEC_IB)
+#else
+# define _TIF_WORK_CTXSW	(_TIF_WORK_CTXSW_BASE)
+#endif
 
 #define _TIF_WORK_CTXSW_PREV (_TIF_WORK_CTXSW|_TIF_USER_RETURN_NOTIFY)
 #define _TIF_WORK_CTXSW_NEXT (_TIF_WORK_CTXSW)
--- a/arch/x86/kernel/process.h
+++ b/arch/x86/kernel/process.h
@@ -2,6 +2,8 @@
 //
 // Code shared between 32 and 64 bit
 
+#include <asm/spec-ctrl.h>
+
 void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p);
 
 /*
@@ -14,6 +16,19 @@ static inline void switch_to_extra(struc
 	unsigned long next_tif = task_thread_info(next)->flags;
 	unsigned long prev_tif = task_thread_info(prev)->flags;
 
+	if (IS_ENABLED(CONFIG_SMP)) {
+		/*
+		 * Avoid __switch_to_xtra() invocation when conditional
+		 * STIPB is disabled and the only different bit is
+		 * TIF_SPEC_IB. For CONFIG_SMP=n TIF_SPEC_IB is not
+		 * in the TIF_WORK_CTXSW masks.
+		 */
+		if (!static_branch_likely(&switch_to_cond_stibp)) {
+			prev_tif &= ~_TIF_SPEC_IB;
+			next_tif &= ~_TIF_SPEC_IB;
+		}
+	}
+
 	/*
 	 * __switch_to_xtra() handles debug registers, i/o bitmaps,
 	 * speculation mitigations etc.