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authorJosh Poimboeuf <jpoimboe@redhat.com>2019-07-04 10:46:37 -0500
committerThomas Gleixner <tglx@linutronix.de>2019-07-22 12:00:51 +0200
commitbe261ffce6f13229dad50f59c5e491f933d3167f (patch)
tree062d6c937566a4c9f3151384a41b4b38a30dc2f8 /tools/arch
parent018ebca8bd704f18d56f8fff38e2c3d76d7d39fb (diff)
x86: Remove X86_FEATURE_MFENCE_RDTSC
AMD and Intel both have serializing lfence (X86_FEATURE_LFENCE_RDTSC). They've both had it for a long time, and AMD has had it enabled in Linux since Spectre v1 was announced. Back then, there was a proposal to remove the serializing mfence feature bit (X86_FEATURE_MFENCE_RDTSC), since both AMD and Intel have serializing lfence. At the time, it was (ahem) speculated that some hypervisors might not yet support its removal, so it remained for the time being. Now a year-and-a-half later, it should be safe to remove. I asked Andrew Cooper about whether it's still needed: So if you're virtualised, you've got no choice in the matter.  lfence is either dispatch-serialising or not on AMD, and you won't be able to change it. Furthermore, you can't accurately tell what state the bit is in, because the MSR might not be virtualised at all, or may not reflect the true state in hardware.  Worse still, attempting to set the bit may not be successful even if there isn't a fault for doing so. Xen sets the DE_CFG bit unconditionally, as does Linux by the looks of things (see MSR_F10H_DECFG_LFENCE_SERIALIZE_BIT).  ISTR other hypervisor vendors saying the same, but I don't have any information to hand. If you are running under a hypervisor which has been updated, then lfence will almost certainly be dispatch-serialising in practice, and you'll almost certainly see the bit already set in DE_CFG.  If you're running under a hypervisor which hasn't been patched since Spectre, you've already lost in many more ways. I'd argue that X86_FEATURE_MFENCE_RDTSC is not worth keeping. So remove it. This will reduce some code rot, and also make it easier to hook barrier_nospec() up to a cmdline disable for performance raisins, without having to need an alternative_3() macro. Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Link: https://lkml.kernel.org/r/d990aa51e40063acb9888e8c1b688e41355a9588.1562255067.git.jpoimboe@redhat.com
Diffstat (limited to 'tools/arch')
-rw-r--r--tools/arch/x86/include/asm/cpufeatures.h1
1 files changed, 0 insertions, 1 deletions
diff --git a/tools/arch/x86/include/asm/cpufeatures.h b/tools/arch/x86/include/asm/cpufeatures.h
index 998c2cc08363..9b98edb6b2d3 100644
--- a/tools/arch/x86/include/asm/cpufeatures.h
+++ b/tools/arch/x86/include/asm/cpufeatures.h
@@ -96,7 +96,6 @@
#define X86_FEATURE_SYSCALL32 ( 3*32+14) /* "" syscall in IA32 userspace */
#define X86_FEATURE_SYSENTER32 ( 3*32+15) /* "" sysenter in IA32 userspace */
#define X86_FEATURE_REP_GOOD ( 3*32+16) /* REP microcode works well */
-#define X86_FEATURE_MFENCE_RDTSC ( 3*32+17) /* "" MFENCE synchronizes RDTSC */
#define X86_FEATURE_LFENCE_RDTSC ( 3*32+18) /* "" LFENCE synchronizes RDTSC */
#define X86_FEATURE_ACC_POWER ( 3*32+19) /* AMD Accumulated Power Mechanism */
#define X86_FEATURE_NOPL ( 3*32+20) /* The NOPL (0F 1F) instructions */