Index: lib/Target/AArch64/AArch64SystemOperands.td
===================================================================
--- lib/Target/AArch64/AArch64SystemOperands.td
+++ lib/Target/AArch64/AArch64SystemOperands.td
@@ -325,6 +325,9 @@
 // v8.5a Spectre Mitigation
 let Requires = [{ {AArch64::FeatureSpecRestrict} }] in
 def : PState<"SSBS",    0b11001>;
+// v8.5a Memory Tagging Extension
+let Requires = [{ {AArch64::FeatureMTE} }] in
+def : PState<"TCO",     0b11100>;
 
 //===----------------------------------------------------------------------===//
 // PSB instruction options.
@@ -1411,6 +1414,19 @@
 let Requires = [{ {AArch64::FeatureSpecRestrict} }] in
 def : RWSysReg<"SSBS", 0b11, 0b011, 0b0100, 0b0010, 0b110>;
 
+// v8.5a Memory Tagging Extension
+//                                 Op0   Op1    CRn     CRm     Op2
+let Requires = [{ {AArch64::FeatureMTE} }] in {
+def : RWSysReg<"TCO",              0b11, 0b011, 0b0100, 0b0010, 0b111>;
+def : RWSysReg<"GCR_EL1",          0b11, 0b000, 0b0001, 0b0000, 0b110>;
+def : RWSysReg<"RGSR_EL1",         0b11, 0b000, 0b0001, 0b0000, 0b101>;
+def : RWSysReg<"TFSR_EL1",         0b11, 0b000, 0b0110, 0b0101, 0b000>;
+def : RWSysReg<"TFSR_EL2",         0b11, 0b100, 0b0110, 0b0101, 0b000>;
+def : RWSysReg<"TFSR_EL3",         0b11, 0b110, 0b0110, 0b0110, 0b000>;
+def : RWSysReg<"TFSR_EL12",        0b11, 0b101, 0b0110, 0b0110, 0b000>;
+def : RWSysReg<"TFSRE0_EL1",       0b11, 0b000, 0b0110, 0b0110, 0b001>;
+} // HasMTE
+
 // Cyclone specific system registers
 //                                 Op0    Op1     CRn     CRm    Op2
 let Requires = [{ {AArch64::ProcCyclone} }] in
Index: test/MC/AArch64/armv8.5a-mte-error.s
===================================================================
--- /dev/null
+++ test/MC/AArch64/armv8.5a-mte-error.s
@@ -0,0 +1,133 @@
+// RUN: not llvm-mc -triple aarch64 -show-encoding -mattr=+mte < %s 2>&1| FileCheck %s
+
+mrs tco
+mrs gcr_el1
+mrs rgsr_el1
+mrs tfsr_el1
+mrs tfsr_el2
+mrs tfsr_el3
+mrs tfsr_el12
+mrs tfsre0_el1
+
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tco
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: gcr_el1
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: rgsr_el1
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tfsr_el1
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tfsr_el2
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tfsr_el3
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tfsr_el12
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tfsre0_el1
+
+mrs tco, #0
+mrs tco, x0
+mrs gcr_el1, x1
+mrs rgsr_el1, x2
+mrs tfsr_el1, x3
+mrs tfsr_el2, x4
+mrs tfsr_el3, x5
+mrs tfsr_el12, x6
+mrs tfsre0_el1, x7
+
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tco, #0
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tco, x0
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: gcr_el1
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: rgsr_el1
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tfsr_el1
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tfsr_el2
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tfsr_el3
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tfsr_el12
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tfsre0_el1
+
+msr tco
+msr gcr_el1
+msr rgsr_el1
+msr tfsr_el1
+msr tfsr_el2
+msr tfsr_el3
+msr tfsr_el12
+msr tfsre0_el1
+
+// CHECK:      too few operands for instruction
+// CHECK-NEXT: tco
+// CHECK:      too few operands for instruction
+// CHECK-NEXT: gcr_el1
+// CHECK:      too few operands for instruction
+// CHECK-NEXT: rgsr_el1
+// CHECK:      too few operands for instruction
+// CHECK-NEXT: tfsr_el1
+// CHECK:      too few operands for instruction
+// CHECK-NEXT: tfsr_el2
+// CHECK:      too few operands for instruction
+// CHECK-NEXT: tfsr_el3
+// CHECK:      too few operands for instruction
+// CHECK-NEXT: tfsr_el12
+// CHECK:      too few operands for instruction
+// CHECK-NEXT: tfsre0_el1
+
+msr x0, tco
+msr x1, gcr_el1
+msr x2, rgsr_el1
+msr x3, tfsr_el1
+msr x4, tfsr_el2
+msr x5, tfsr_el3
+msr x6, tfsr_el12
+msr x7, tfsre0_el1
+
+// CHECK:      expected writable system register or pstate
+// CHECK-NEXT: tco
+// CHECK:      expected writable system register or pstate
+// CHECK-NEXT: gcr_el1
+// CHECK:      expected writable system register or pstate
+// CHECK-NEXT: rgsr_el1
+// CHECK:      expected writable system register or pstate
+// CHECK-NEXT: tfsr_el1
+// CHECK:      expected writable system register or pstate
+// CHECK-NEXT: tfsr_el2
+// CHECK:      expected writable system register or pstate
+// CHECK-NEXT: tfsr_el3
+// CHECK:      expected writable system register or pstate
+// CHECK-NEXT: tfsr_el12
+// CHECK:      expected writable system register or pstate
+// CHECK-NEXT: tfsre0_el1
+
+// Among the system registers added by MTE, only TCO can be used with MSR (imm).
+// The rest can only be used with MSR (reg).
+msr gcr_el1, #1
+msr rgsr_el1, #2
+msr tfsr_el1, #3
+msr tfsr_el2, #4
+msr tfsr_el3, #5
+msr tfsr_el12, #6
+msr tfsre0_el1, #7
+
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: gcr_el1
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: rgsr_el1
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tfsr_el1
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tfsr_el2
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tfsr_el3
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tfsr_el12
+// CHECK:      invalid operand for instruction
+// CHECK-NEXT: tfsre0_el1
Index: test/MC/AArch64/armv8.5a-mte.s
===================================================================
--- test/MC/AArch64/armv8.5a-mte.s
+++ test/MC/AArch64/armv8.5a-mte.s
@@ -58,3 +58,80 @@
 // NOMTE: DC CGDVADP requires mte
 // NOMTE: DC CIGDVAC requires mte
 // NOMTE: DC GZVA requires mte
+
+mrs x0, tco
+mrs x1, gcr_el1
+mrs x2, rgsr_el1
+mrs x3, tfsr_el1
+mrs x4, tfsr_el2
+mrs x5, tfsr_el3
+mrs x6, tfsr_el12
+mrs x7, tfsre0_el1
+
+// CHECK: mrs x0, TCO           // encoding: [0xe0,0x42,0x3b,0xd5]
+// CHECK: mrs x1, GCR_EL1       // encoding: [0xc1,0x10,0x38,0xd5]
+// CHECK: mrs x2, RGSR_EL1      // encoding: [0xa2,0x10,0x38,0xd5]
+// CHECK: mrs x3, TFSR_EL1      // encoding: [0x03,0x65,0x38,0xd5]
+// CHECK: mrs x4, TFSR_EL2      // encoding: [0x04,0x65,0x3c,0xd5]
+// CHECK: mrs x5, TFSR_EL3      // encoding: [0x05,0x66,0x3e,0xd5]
+// CHECK: mrs x6, TFSR_EL12     // encoding: [0x06,0x66,0x3d,0xd5]
+// CHECK: mrs x7, TFSRE0_EL1    // encoding: [0x27,0x66,0x38,0xd5]
+
+// NOMTE: expected readable system register
+// NOMTE-NEXT: tco
+// NOMTE: expected readable system register
+// NOMTE-NEXT: gcr_el1
+// NOMTE: expected readable system register
+// NOMTE-NEXT: rgsr_el1
+// NOMTE: expected readable system register
+// NOMTE-NEXT: tfsr_el1
+// NOMTE: expected readable system register
+// NOMTE-NEXT: tfsr_el2
+// NOMTE: expected readable system register
+// NOMTE-NEXT: tfsr_el3
+// NOMTE: expected readable system register
+// NOMTE-NEXT: tfsr_el12
+// NOMTE: expected readable system register
+// NOMTE-NEXT: tfsre0_el1
+
+msr tco, #0
+
+// CHECK: msr TCO, #0           // encoding: [0x9f,0x40,0x03,0xd5]
+
+// NOMTE: expected writable system register or pstate
+// NOMTE-NEXT: tco
+
+msr tco, x0
+msr gcr_el1, x1
+msr rgsr_el1, x2
+msr tfsr_el1, x3
+msr tfsr_el2, x4
+msr tfsr_el3, x5
+msr tfsr_el12, x6
+msr tfsre0_el1, x7
+
+// CHECK: msr TCO, x0           // encoding: [0xe0,0x42,0x1b,0xd5]
+// CHECK: msr GCR_EL1, x1       // encoding: [0xc1,0x10,0x18,0xd5]
+// CHECK: msr RGSR_EL1, x2      // encoding: [0xa2,0x10,0x18,0xd5]
+// CHECK: msr TFSR_EL1, x3      // encoding: [0x03,0x65,0x18,0xd5]
+// CHECK: msr TFSR_EL2, x4      // encoding: [0x04,0x65,0x1c,0xd5]
+// CHECK: msr TFSR_EL3, x5      // encoding: [0x05,0x66,0x1e,0xd5]
+// CHECK: msr TFSR_EL12, x6     // encoding: [0x06,0x66,0x1d,0xd5]
+// CHECK: msr TFSRE0_EL1, x7    // encoding: [0x27,0x66,0x18,0xd5]
+
+// NOMTE: expected writable system register or pstate
+// NOMTE-NEXT: tco
+// NOMTE: expected writable system register or pstate
+// NOMTE-NEXT: gcr_el1
+// NOMTE: expected writable system register or pstate
+// NOMTE-NEXT: rgsr_el1
+// NOMTE: expected writable system register or pstate
+// NOMTE-NEXT: tfsr_el1
+// NOMTE: expected writable system register or pstate
+// NOMTE-NEXT: tfsr_el2
+// NOMTE: expected writable system register or pstate
+// NOMTE-NEXT: tfsr_el3
+// NOMTE: expected writable system register or pstate
+// NOMTE-NEXT: tfsr_el12
+// NOMTE: expected writable system register or pstate
+// NOMTE-NEXT: tfsre0_el1
Index: test/MC/Disassembler/AArch64/armv8.5a-mte.txt
===================================================================
--- test/MC/Disassembler/AArch64/armv8.5a-mte.txt
+++ test/MC/Disassembler/AArch64/armv8.5a-mte.txt
@@ -1,6 +1,6 @@
-# RUN: llvm-mc -triple=aarch64 -mattr=+mte  -disassemble < %s | FileCheck %s
+# RUN: llvm-mc -triple=aarch64 -mattr=+mte   -disassemble < %s | FileCheck %s
 # RUN: llvm-mc -triple=aarch64 -mattr=+v8.5a -disassemble < %s | FileCheck %s --check-prefix=NOMTE
-# RUN: llvm-mc -triple=aarch64 -mattr=-mte  -disassemble < %s | FileCheck %s --check-prefix=NOMTE
+# RUN: llvm-mc -triple=aarch64 -mattr=-mte   -disassemble < %s | FileCheck %s --check-prefix=NOMTE
 
 [0x60,0x76,0x08,0xd5]
 [0x81,0x76,0x08,0xd5]
@@ -58,3 +58,43 @@
 # NOMTE: sys #3, c7, c13, #5, x15
 # NOMTE: sys #3, c7, c14, #5, x16
 # NOMTE: sys #3, c7, c4, #4, x17
+
+[0xe0,0x42,0x3b,0xd5]
+[0xc1,0x10,0x38,0xd5]
+[0xa2,0x10,0x38,0xd5]
+[0x03,0x65,0x38,0xd5]
+[0x04,0x65,0x3c,0xd5]
+[0x05,0x66,0x3e,0xd5]
+[0x06,0x66,0x3d,0xd5]
+[0x27,0x66,0x38,0xd5]
+
+# CHECK: mrs x0, TCO
+# CHECK: mrs x1, GCR_EL1
+# CHECK: mrs x2, RGSR_EL1
+# CHECK: mrs x3, TFSR_EL1
+# CHECK: mrs x4, TFSR_EL2
+# CHECK: mrs x5, TFSR_EL3
+# CHECK: mrs x6, TFSR_EL12
+# CHECK: mrs x7, TFSRE0_EL1
+
+[0x9f,0x40,0x03,0xd5]
+
+# CHECK: msr TCO, #0
+
+[0xe0,0x42,0x1b,0xd5]
+[0xc1,0x10,0x18,0xd5]
+[0xa2,0x10,0x18,0xd5]
+[0x03,0x65,0x18,0xd5]
+[0x04,0x65,0x1c,0xd5]
+[0x05,0x66,0x1e,0xd5]
+[0x06,0x66,0x1d,0xd5]
+[0x27,0x66,0x18,0xd5]
+
+# CHECK: msr TCO, x0
+# CHECK: msr GCR_EL1, x1
+# CHECK: msr RGSR_EL1, x2
+# CHECK: msr TFSR_EL1, x3
+# CHECK: msr TFSR_EL2, x4
+# CHECK: msr TFSR_EL3, x5
+# CHECK: msr TFSR_EL12, x6
+# CHECK: msr TFSRE0_EL1, x7