Index: clang/test/Preprocessor/riscv-target-features.c
===================================================================
--- clang/test/Preprocessor/riscv-target-features.c
+++ clang/test/Preprocessor/riscv-target-features.c
@@ -462,3 +462,11 @@
 // RUN: %clang -target riscv64 -march=rv64izicbop -x c -E -dM %s \
 // RUN: -o - | FileCheck --check-prefix=CHECK-ZICBOP-EXT %s
 // CHECK-ZICBOP-EXT: __riscv_zicbop 1000000{{$}}
+
+// RUN: %clang -target riscv32-unknown-linux-gnu -menable-experimental-extensions \
+// RUN: -march=rv32iztso0p1 -x c -E -dM %s \
+// RUN: -o - | FileCheck --check-prefix=CHECK-ZTSO-EXT %s
+// RUN: %clang -target riscv64-unknown-linux-gnu -menable-experimental-extensions \
+// RUN: -march=rv64iztso0p1 -x c -E -dM %s \
+// RUN: -o - | FileCheck --check-prefix=CHECK-ZTSO-EXT %s
+// CHECK-ZTSO-EXT: __riscv_ztso 1000{{$}}
Index: llvm/lib/Support/RISCVISAInfo.cpp
===================================================================
--- llvm/lib/Support/RISCVISAInfo.cpp
+++ llvm/lib/Support/RISCVISAInfo.cpp
@@ -114,6 +114,7 @@
     {"zbt", RISCVExtensionVersion{0, 93}},
     {"zca", RISCVExtensionVersion{0, 70}},
     {"zvfh", RISCVExtensionVersion{0, 1}},
+    {"ztso", RISCVExtensionVersion{0, 1}},
 };
 
 static bool stripExperimentalPrefix(StringRef &Ext) {
Index: llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.cpp
===================================================================
--- llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.cpp
+++ llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.cpp
@@ -157,6 +157,8 @@
 
   if (Features[RISCV::FeatureStdExtC])
     EFlags |= ELF::EF_RISCV_RVC;
+  if (Features[RISCV::FeatureStdExtZtso])
+    EFlags |= ELF::EF_RISCV_TSO;
 
   switch (ABI) {
   case RISCVABI::ABI_ILP32:
Index: llvm/lib/Target/RISCV/RISCV.td
===================================================================
--- llvm/lib/Target/RISCV/RISCV.td
+++ llvm/lib/Target/RISCV/RISCV.td
@@ -454,6 +454,13 @@
                                 AssemblerPredicate<(all_of FeatureStdExtZicbop),
                                 "'Zicbop' (Cache-Block Prefetch Instructions)">;
 
+def FeatureStdExtZtso
+    : SubtargetFeature<"experimental-ztso", "HasStdExtZtso", "true",
+                       "'Ztso' (Memory Model - Total Store Order)">;
+def HasStdExtZtso : Predicate<"Subtarget->hasStdExtZTso()">,
+                           AssemblerPredicate<(all_of FeatureStdExtZtso),
+                           "'Ztso' (Memory Model - Total Store Order)">;
+
 // Feature32Bit exists to mark CPUs that support RV32 to distinquish them from
 // tuning CPU names.
 def Feature32Bit
Index: llvm/lib/Target/RISCV/RISCVSubtarget.h
===================================================================
--- llvm/lib/Target/RISCV/RISCVSubtarget.h
+++ llvm/lib/Target/RISCV/RISCVSubtarget.h
@@ -92,6 +92,7 @@
   bool HasStdExtZicboz = false;
   bool HasStdExtZicbop = false;
   bool HasStdExtZmmul = false;
+  bool HasStdExtZtso = false;
   bool HasRV32 = false;
   bool HasRV64 = false;
   bool IsRV32E = false;
@@ -192,6 +193,7 @@
   bool hasStdExtZicboz() const { return HasStdExtZicboz; }
   bool hasStdExtZicbop() const { return HasStdExtZicbop; }
   bool hasStdExtZmmul() const { return HasStdExtZmmul; }
+  bool hasStdExtZtso() const { return HasStdExtZtso; }
   bool is64Bit() const { return HasRV64; }
   bool isRV32E() const { return IsRV32E; }
   bool enableLinkerRelax() const { return EnableLinkerRelax; }
Index: llvm/test/CodeGen/RISCV/attributes.ll
===================================================================
--- llvm/test/CodeGen/RISCV/attributes.ll
+++ llvm/test/CodeGen/RISCV/attributes.ll
@@ -84,6 +84,7 @@
 ; RUN: llc -mtriple=riscv64 -mattr=+zicbom %s -o - | FileCheck --check-prefix=RV64ZICBOM %s
 ; RUN: llc -mtriple=riscv64 -mattr=+zicboz %s -o - | FileCheck --check-prefix=RV64ZICBOZ %s
 ; RUN: llc -mtriple=riscv64 -mattr=+zicbop %s -o - | FileCheck --check-prefix=RV64ZICBOP %s
+; RUN: llc -mtriple=riscv64 -mattr=+experimental-ztso %s -o - | FileCheck --check-prefix=RV64ZTSO %s
 
 ; RV32M: .attribute 5, "rv32i2p0_m2p0"
 ; RV32ZMMUL: .attribute 5, "rv32i2p0_zmmul1p0"
@@ -170,6 +171,7 @@
 ; RV64ZICBOM: .attribute 5, "rv64i2p0_zicbom1p0"
 ; RV64ZICBOZ: .attribute 5, "rv64i2p0_zicboz1p0"
 ; RV64ZICBOP: .attribute 5, "rv64i2p0_zicbop1p0"
+; RV64ZTSO: .attribute 5, "rv64i2p0_ztso0p1"
 
 define i32 @addi(i32 %a) {
   %1 = add i32 %a, 1
Index: llvm/test/MC/RISCV/attribute-arch.s
===================================================================
--- llvm/test/MC/RISCV/attribute-arch.s
+++ llvm/test/MC/RISCV/attribute-arch.s
@@ -196,3 +196,6 @@
 
 .attribute arch, "rv32izca0p70"
 # CHECK: attribute      5, "rv32i2p0_zca0p70"
+
+.attribute arch, "rv32iztso0p1"
+# CHECK: attribute      5, "rv32i2p0_ztso0p1"
Index: llvm/test/MC/RISCV/elf-flags.s
===================================================================
--- llvm/test/MC/RISCV/elf-flags.s
+++ llvm/test/MC/RISCV/elf-flags.s
@@ -5,6 +5,8 @@
 # RUN: llvm-mc -triple=riscv32 -mattr=+e -filetype=obj < %s \
 # RUN:   | llvm-readobj --file-headers - \
 # RUN:   | FileCheck -check-prefix=CHECK-RVE %s
+# RUN: llvm-mc -triple=riscv32 -mattr=+experimental-ztso -filetype=obj < %s | llvm-readobj --file-headers - | FileCheck -check-prefixes=CHECK-TSO %s
+# RUN: llvm-mc -triple=riscv64 -mattr=+experimental-ztso -filetype=obj < %s | llvm-readobj --file-headers - | FileCheck -check-prefixes=CHECK-TSO %s
 
 # CHECK-RVI:       Flags [ (0x0)
 # CHECK-RVI-NEXT:  ]
@@ -17,4 +19,8 @@
 # CHECK-RVE-NEXT:     EF_RISCV_RVE (0x8)
 # CHECK-RVE-NEXT:   ]
 
+# CHECK-TSO:        Flags [ (0x10)
+# CHECK-NEXT-TSO      EF_RISCV_TSO (0x10)
+# CHECK-NEXT-TSO    ]
+
 nop