linux-kernel - [tip:x86/pti] x86/cpu/AMD: Use LFENCE_RDTSC in preference to MFENCE

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Date:   Mon, 8 Jan 2018 16:49:16 -0800
From:   tip-bot for Tom Lendacky <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     dwmw@...zon.co.uk, tglx@...utronix.de, bp@...e.de, pjt@...gle.com,
        thomas.lendacky@....com, gregkh@...ux-foundation.org,
        peterz@...radead.org, tim.c.chen@...ux.intel.com,
        dave.hansen@...el.com, linux-kernel@...r.kernel.org,
        dan.j.williams@...el.com, torvalds@...ux-foundation.org,
        bp@...en8.de, mingo@...nel.org, hpa@...or.com
Subject: [tip:x86/pti] x86/cpu/AMD: Use LFENCE_RDTSC in preference to
 MFENCE_RDTSC

Commit-ID:  9c6a73c75864ad9fa49e5fa6513e4c4071c0e29f
Gitweb:     https://git.kernel.org/tip/9c6a73c75864ad9fa49e5fa6513e4c4071c0e29f
Author:     Tom Lendacky <thomas.lendacky@....com>
AuthorDate: Mon, 8 Jan 2018 16:09:32 -0600
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Tue, 9 Jan 2018 01:43:11 +0100

x86/cpu/AMD: Use LFENCE_RDTSC in preference to MFENCE_RDTSC

With LFENCE now a serializing instruction, use LFENCE_RDTSC in preference
to MFENCE_RDTSC.  However, since the kernel could be running under a
hypervisor that does not support writing that MSR, read the MSR back and
verify that the bit has been set successfully.  If the MSR can be read
and the bit is set, then set the LFENCE_RDTSC feature, otherwise set the
MFENCE_RDTSC feature.

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/20180108220932.12580.52458.stgit@tlendack-t1.amdoffice.net

---
 arch/x86/include/asm/msr-index.h |  1 +
 arch/x86/kernel/cpu/amd.c        | 18 ++++++++++++++++--
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 1e7d710..fa11fb1 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -354,6 +354,7 @@
 #define MSR_FAM10H_NODE_ID		0xc001100c
 #define MSR_F10H_DECFG			0xc0011029
 #define MSR_F10H_DECFG_LFENCE_SERIALIZE_BIT	1
+#define MSR_F10H_DECFG_LFENCE_SERIALIZE		BIT_ULL(MSR_F10H_DECFG_LFENCE_SERIALIZE_BIT)
 
 /* 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 5b438d8..ea831c8 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -829,6 +829,9 @@ static void init_amd(struct cpuinfo_x86 *c)
 		set_cpu_cap(c, X86_FEATURE_K8);
 
 	if (cpu_has(c, X86_FEATURE_XMM2)) {
+		unsigned long long val;
+		int ret;
+
 		/*
 		 * A serializing LFENCE has less overhead than MFENCE, so
 		 * use it for execution serialization.  On families which
@@ -839,8 +842,19 @@ static void init_amd(struct cpuinfo_x86 *c)
 		msr_set_bit(MSR_F10H_DECFG,
 			    MSR_F10H_DECFG_LFENCE_SERIALIZE_BIT);
 
-		/* MFENCE stops RDTSC speculation */
-		set_cpu_cap(c, X86_FEATURE_MFENCE_RDTSC);
+		/*
+		 * Verify that the MSR write was successful (could be running
+		 * under a hypervisor) and only then assume that LFENCE is
+		 * serializing.
+		 */
+		ret = rdmsrl_safe(MSR_F10H_DECFG, &val);
+		if (!ret && (val & MSR_F10H_DECFG_LFENCE_SERIALIZE)) {
+			/* A serializing LFENCE stops RDTSC speculation */
+			set_cpu_cap(c, X86_FEATURE_LFENCE_RDTSC);
+		} else {
+			/* MFENCE stops RDTSC speculation */
+			set_cpu_cap(c, X86_FEATURE_MFENCE_RDTSC);
+		}
 	}
 
 	/*