linux-kernel - [tip:x86/pti] x86/cpu/AMD: Make LFENCE a serializing instruction

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Date:   Mon, 8 Jan 2018 16:48:48 -0800
From:   tip-bot for Tom Lendacky <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     hpa@...or.com, dave.hansen@...el.com, thomas.lendacky@....com,
        tglx@...utronix.de, bp@...en8.de, peterz@...radead.org,
        torvalds@...ux-foundation.org, dan.j.williams@...el.com,
        gregkh@...ux-foundation.org, dwmw@...zon.co.uk, mingo@...nel.org,
        pjt@...gle.com, tim.c.chen@...ux.intel.com, bp@...e.de,
        linux-kernel@...r.kernel.org
Subject: [tip:x86/pti] x86/cpu/AMD: Make LFENCE a serializing instruction

Commit-ID:  e4d0e84e490790798691aaa0f2e598637f1867ec
Gitweb:     https://git.kernel.org/tip/e4d0e84e490790798691aaa0f2e598637f1867ec
Author:     Tom Lendacky <thomas.lendacky@....com>
AuthorDate: Mon, 8 Jan 2018 16:09:21 -0600
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Tue, 9 Jan 2018 01:43:10 +0100

x86/cpu/AMD: Make LFENCE a serializing instruction

To aid in speculation control, make LFENCE a serializing instruction
since it has less overhead than MFENCE.  This is done by setting bit 1
of MSR 0xc0011029 (DE_CFG).  Some families that support LFENCE do not
have this MSR.  For these families, the LFENCE instruction is already
serializing.

Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Reviewed-by: Reviewed-by: Borislav Petkov <bp@...e.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: Dave Hansen <dave.hansen@...el.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dan Williams <dan.j.williams@...el.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@...ux-foundation.org>
Cc: David Woodhouse <dwmw@...zon.co.uk>
Cc: Paul Turner <pjt@...gle.com>
Link: https://lkml.kernel.org/r/20180108220921.12580.71694.stgit@tlendack-t1.amdoffice.net

---
 arch/x86/include/asm/msr-index.h |  2 ++
 arch/x86/kernel/cpu/amd.c        | 10 ++++++++++
 2 files changed, 12 insertions(+)

diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index ab02261..1e7d710 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -352,6 +352,8 @@
 #define FAM10H_MMIO_CONF_BASE_MASK	0xfffffffULL
 #define FAM10H_MMIO_CONF_BASE_SHIFT	20
 #define MSR_FAM10H_NODE_ID		0xc001100c
+#define MSR_F10H_DECFG			0xc0011029
+#define MSR_F10H_DECFG_LFENCE_SERIALIZE_BIT	1
 
 /* K8 MSRs */
 #define MSR_K8_TOP_MEM1			0xc001001a
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index bcb75dc..5b438d8 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -829,6 +829,16 @@ static void init_amd(struct cpuinfo_x86 *c)
 		set_cpu_cap(c, X86_FEATURE_K8);
 
 	if (cpu_has(c, X86_FEATURE_XMM2)) {
+		/*
+		 * A serializing LFENCE has less overhead than MFENCE, so
+		 * use it for execution serialization.  On families which
+		 * don't have that MSR, LFENCE is already serializing.
+		 * msr_set_bit() uses the safe accessors, too, even if the MSR
+		 * is not present.
+		 */
+		msr_set_bit(MSR_F10H_DECFG,
+			    MSR_F10H_DECFG_LFENCE_SERIALIZE_BIT);
+
 		/* MFENCE stops RDTSC speculation */
 		set_cpu_cap(c, X86_FEATURE_MFENCE_RDTSC);
 	}