wasm2c: Segue support for CPUs without wrgsbase instructions

3 files changed, 63 insertions, 15 deletions

diff --git a/src/c-writer.cc b/src/c-writer.cc
index 1a091957..986ace55 100644
--- a/src/c-writer.cc
+++ b/src/c-writer.cc
@@ -2449,17 +2449,17 @@ bool CWriter::IsSingleUnsharedMemory() {
 void CWriter::InstallSegueBase(Memory* memory, bool save_old_value) {
   NonIndented([&] { Write("#if WASM_RT_USE_SEGUE", Newline()); });
   if (save_old_value) {
-    Write("uintptr_t segue_saved_base = WASM_RT_SEGUE_READ_BASE();", Newline());
+    Write("void* segue_saved_base = wasm_rt_segue_read_base();", Newline());
   }
   auto primary_memory =
       ExternalInstanceRef(ModuleFieldType::Memory, memory->name);
-  Write("WASM_RT_SEGUE_WRITE_BASE(", primary_memory, ".data);", Newline());
+  Write("wasm_rt_segue_write_base(", primary_memory, ".data);", Newline());
   NonIndented([&] { Write("#endif", Newline()); });
 }
 
 void CWriter::RestoreSegueBase() {
   NonIndented([&] { Write("#if WASM_RT_USE_SEGUE", Newline()); });
-  Write("WASM_RT_SEGUE_WRITE_BASE(segue_saved_base);", Newline());
+  Write("wasm_rt_segue_write_base(segue_saved_base);", Newline());
   NonIndented([&] { Write("#endif", Newline()); });
 }
 
diff --git a/src/prebuilt/wasm2c_source_declarations.cc b/src/prebuilt/wasm2c_source_declarations.cc
index 11aac524..000b25c8 100644
--- a/src/prebuilt/wasm2c_source_declarations.cc
+++ b/src/prebuilt/wasm2c_source_declarations.cc
@@ -64,15 +64,25 @@ R"w2c_template(//     "address namespaces". GCC does not support the memcpy requ
 )w2c_template"
 R"w2c_template(//     this leaves only clang for now.
 )w2c_template"
-R"w2c_template(// (5) The OS doesn't replace the segment register on context switch which
+R"w2c_template(// (5) The OS provides a way to query if (rd|wr)gsbase is allowed by the kernel
+)w2c_template"
+R"w2c_template(// or the implementation has to use a syscall for this.
+)w2c_template"
+R"w2c_template(// (6) The OS doesn't replace the segment register on context switch which
 )w2c_template"
 R"w2c_template(//     eliminates windows for now
 )w2c_template"
+R"w2c_template(//
+)w2c_template"
+R"w2c_template(// While more OS can be supported in the future, we only support linux for now
+)w2c_template"
 R"w2c_template(#if WASM_RT_ALLOW_SEGUE && !WABT_BIG_ENDIAN &&                               \
 )w2c_template"
 R"w2c_template(    (defined(__x86_64__) || defined(_M_X64)) && IS_SINGLE_UNSHARED_MEMORY && \
 )w2c_template"
-R"w2c_template(    __clang__ && __has_builtin(__builtin_ia32_wrgsbase64) && !defined(_WIN32)
+R"w2c_template(    __clang__ && __has_builtin(__builtin_ia32_wrgsbase64) &&                 \
+)w2c_template"
+R"w2c_template(    !defined(_WIN32) && defined(__linux__)
 )w2c_template"
 R"w2c_template(#define WASM_RT_USE_SEGUE 1
 )w2c_template"
@@ -89,11 +99,33 @@ R"w2c_template(
 )w2c_template"
 R"w2c_template(// POSIX uses FS for TLS, GS is free
 )w2c_template"
-R"w2c_template(#define WASM_RT_SEGUE_READ_BASE() __builtin_ia32_rdgsbase64()
+R"w2c_template(static inline void* wasm_rt_segue_read_base() {
 )w2c_template"
-R"w2c_template(#define WASM_RT_SEGUE_WRITE_BASE(base) \
+R"w2c_template(  if (wasm_rt_fsgsbase_inst_supported) {
+)w2c_template"
+R"w2c_template(    return (void*)__builtin_ia32_rdgsbase64();
+)w2c_template"
+R"w2c_template(  } else {
+)w2c_template"
+R"w2c_template(    return wasm_rt_syscall_get_segue_base();
+)w2c_template"
+R"w2c_template(  }
 )w2c_template"
-R"w2c_template(  __builtin_ia32_wrgsbase64((uintptr_t)base)
+R"w2c_template(}
+)w2c_template"
+R"w2c_template(static inline void wasm_rt_segue_write_base(void* base) {
+)w2c_template"
+R"w2c_template(  if (wasm_rt_fsgsbase_inst_supported) {
+)w2c_template"
+R"w2c_template(    __builtin_ia32_wrgsbase64((uintptr_t)base);
+)w2c_template"
+R"w2c_template(  } else {
+)w2c_template"
+R"w2c_template(    wasm_rt_syscall_set_segue_base(base);
+)w2c_template"
+R"w2c_template(  }
+)w2c_template"
+R"w2c_template(}
 )w2c_template"
 R"w2c_template(#define MEM_ADDR_MEMOP(mem, addr, n) ((uint8_t __seg_gs*)(uintptr_t)addr)
 )w2c_template"
@@ -194,7 +226,7 @@ R"w2c_template(#include <stdio.h>
 )w2c_template"
 R"w2c_template(#define WASM_RT_CHECK_BASE(mem)                                               \
 )w2c_template"
-R"w2c_template(  if (((uintptr_t)((mem)->data)) != ((uintptr_t)WASM_RT_SEGUE_READ_BASE())) { \
+R"w2c_template(  if (((uintptr_t)((mem)->data)) != ((uintptr_t)wasm_rt_segue_read_base())) { \
 )w2c_template"
 R"w2c_template(    puts("Segment register mismatch\n");                                      \
 )w2c_template"
diff --git a/src/template/wasm2c.declarations.c b/src/template/wasm2c.declarations.c
index 5261a25b..efe9dbbe 100644
--- a/src/template/wasm2c.declarations.c
+++ b/src/template/wasm2c.declarations.c
@@ -32,11 +32,16 @@
 //     for accessing pointers, and supports memcpy on pointers with custom
 //     "address namespaces". GCC does not support the memcpy requirement, so
 //     this leaves only clang for now.
-// (5) The OS doesn't replace the segment register on context switch which
+// (5) The OS provides a way to query if (rd|wr)gsbase is allowed by the kernel
+// or the implementation has to use a syscall for this.
+// (6) The OS doesn't replace the segment register on context switch which
 //     eliminates windows for now
+//
+// While more OS can be supported in the future, we only support linux for now
 #if WASM_RT_ALLOW_SEGUE && !WABT_BIG_ENDIAN &&                               \
     (defined(__x86_64__) || defined(_M_X64)) && IS_SINGLE_UNSHARED_MEMORY && \
-    __clang__ && __has_builtin(__builtin_ia32_wrgsbase64) && !defined(_WIN32)
+    __clang__ && __has_builtin(__builtin_ia32_wrgsbase64) &&                 \
+    !defined(_WIN32) && defined(__linux__)
 #define WASM_RT_USE_SEGUE 1
 #else
 #define WASM_RT_USE_SEGUE 0
@@ -45,9 +50,20 @@
 
 #if WASM_RT_USE_SEGUE
 // POSIX uses FS for TLS, GS is free
-#define WASM_RT_SEGUE_READ_BASE() __builtin_ia32_rdgsbase64()
-#define WASM_RT_SEGUE_WRITE_BASE(base) \
-  __builtin_ia32_wrgsbase64((uintptr_t)base)
+static inline void* wasm_rt_segue_read_base() {
+  if (wasm_rt_fsgsbase_inst_supported) {
+    return (void*)__builtin_ia32_rdgsbase64();
+  } else {
+    return wasm_rt_syscall_get_segue_base();
+  }
+}
+static inline void wasm_rt_segue_write_base(void* base) {
+  if (wasm_rt_fsgsbase_inst_supported) {
+    __builtin_ia32_wrgsbase64((uintptr_t)base);
+  } else {
+    wasm_rt_syscall_set_segue_base(base);
+  }
+}
 #define MEM_ADDR_MEMOP(mem, addr, n) ((uint8_t __seg_gs*)(uintptr_t)addr)
 #else
 #define MEM_ADDR_MEMOP(mem, addr, n) MEM_ADDR(mem, addr, n)
@@ -103,7 +119,7 @@ static inline bool func_types_eq(const wasm_rt_func_type_t a,
 #if WASM_RT_USE_SEGUE && WASM_RT_SANITY_CHECKS
 #include <stdio.h>
 #define WASM_RT_CHECK_BASE(mem)                                               \
-  if (((uintptr_t)((mem)->data)) != ((uintptr_t)WASM_RT_SEGUE_READ_BASE())) { \
+  if (((uintptr_t)((mem)->data)) != ((uintptr_t)wasm_rt_segue_read_base())) { \
     puts("Segment register mismatch\n");                                      \
     abort();                                                                  \
   }