diff --git a/llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp b/llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
--- a/llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
+++ b/llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
@@ -2916,6 +2916,7 @@
     {"ls64", {AArch64::FeatureLS64}},
     {"xs", {AArch64::FeatureXS}},
     {"pauth", {AArch64::FeaturePAuth}},
+    {"flagm", {AArch64::FeatureFlagM}},
     // FIXME: Unsupported extensions
     {"pan", {}},
     {"lor", {}},
diff --git a/llvm/test/MC/AArch64/armv8.4a-flagm.s b/llvm/test/MC/AArch64/armv8.4a-flagm.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/AArch64/armv8.4a-flagm.s
@@ -0,0 +1,6 @@
+// RUN: llvm-mc -triple aarch64-none-linux-gnu -show-encoding -mattr=+flagm   < %s | FileCheck %s
+
+cfinv
+
+// CHECK: .text
+cfinv // encoding: [0x1f,0x40,0x00,0xd5]
diff --git a/llvm/test/MC/AArch64/directive-arch_extension-negative.s b/llvm/test/MC/AArch64/directive-arch_extension-negative.s
--- a/llvm/test/MC/AArch64/directive-arch_extension-negative.s
+++ b/llvm/test/MC/AArch64/directive-arch_extension-negative.s
@@ -1,5 +1,5 @@
 // RUN: not llvm-mc -triple aarch64 \
-// RUN: -mattr=+crc,+sm4,+sha3,+sha2,+aes,+fp,+neon,+ras,+lse,+predres,+ccdp,+mte,+tlb-rmi,+pan-rwv,+ccpp,+rcpc,+ls64 \
+// RUN: -mattr=+crc,+sm4,+sha3,+sha2,+aes,+fp,+neon,+ras,+lse,+predres,+ccdp,+mte,+tlb-rmi,+pan-rwv,+ccpp,+rcpc,+ls64,+flagm \
 // RUN: -filetype asm -o - %s 2>&1 | FileCheck %s
 
 .arch_extension axp64
@@ -124,3 +124,10 @@
 ld64b x0, [x13]
 // CHECK: [[@LINE-1]]:1: error: instruction requires: ls64
 // CHECK-NEXT: ld64b x0, [x13]
+
+cfinv
+// CHECK-NOT: [[@LINE-1]]:1: error: instruction requires: flagm
+.arch_extension noflagm
+cfinv
+// CHECK: [[@LINE-1]]:1: error: instruction requires: flagm
+// CHECK-NEXT: cfinv
diff --git a/llvm/test/MC/AArch64/directive-arch_extension.s b/llvm/test/MC/AArch64/directive-arch_extension.s
--- a/llvm/test/MC/AArch64/directive-arch_extension.s
+++ b/llvm/test/MC/AArch64/directive-arch_extension.s
@@ -75,3 +75,7 @@
 .arch_extension pauth
 paciasp
 // CHECK: paciasp
+
+.arch_extension flagm
+cfinv
+// CHECK: cfinv