summaryrefslogtreecommitdiffstats
path: root/arch/x86/include/asm/barrier.h
diff options
context:
space:
mode:
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 /arch/x86/include/asm/barrier.h
parent018ebca8bd704f18d56f8fff38e2c3d76d7d39fb (diff)
downloadlinux-be261ffce6f13229dad50f59c5e491f933d3167f.tar.bz2
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 'arch/x86/include/asm/barrier.h')
-rw-r--r--arch/x86/include/asm/barrier.h3
1 files changed, 1 insertions, 2 deletions
diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
index 84f848c2541a..7f828fe49797 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -49,8 +49,7 @@ static inline unsigned long array_index_mask_nospec(unsigned long index,
#define array_index_mask_nospec array_index_mask_nospec
/* Prevent speculative execution past this barrier. */
-#define barrier_nospec() alternative_2("", "mfence", X86_FEATURE_MFENCE_RDTSC, \
- "lfence", X86_FEATURE_LFENCE_RDTSC)
+#define barrier_nospec() alternative("", "lfence", X86_FEATURE_LFENCE_RDTSC)
#define dma_rmb() barrier()
#define dma_wmb() barrier()