runtime,cmd/internal/obj/x86: use TEB TLS slots on windows/amd64

This CL redesign how we get the TLS pointer on windows/amd64.

We were previously reading it from the [TEB] arbitrary data slot,
located at 0x28(GS), which can only hold 1 TLS pointer.

With this CL, we will read the TLS pointer from the TEB TLS slot array,
located at 0x1480(GS). The TLS slot array can hold multiple
TLS pointers, up to 64, so multiple Go runtimes running on the
same thread can coexists with different TLS.

Each new TLS slot has to be allocated via [TlsAlloc],
which returns the slot index. This index can then be used to get the
slot offset from GS with the following formula: 0x1480 + index*8

The slot index is fixed per Go runtime, so we can store it
in runtime.tls_g and use it latter on to read/update the TLS pointer.

Loading the TLS pointer requires the following asm instructions:

  MOVQ runtime.tls_g, AX
  MOVQ AX(GS), AX

Notice that this approach is also implemented on windows/arm64.

[TEB]: https://en.wikipedia.org/wiki/Win32_Thread_Information_Block
[TlsAlloc]: https://learn.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-tlsalloc

Updates #22192

Change-Id: Idea7119fd76a3cd083979a4d57ed64b552fa101b
Reviewed-on: https://go-review.googlesource.com/c/go/+/431775
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/cmd/internal/obj/x86/asm6.go

@@ -3612,7 +3612,7 @@ func (ab *AsmBuf) asmandsz(ctxt *obj.Link, cursym *obj.LSym, p *obj.Prog, a *obj
 		goto bad
 	}
 
-	if a.Index != REG_NONE && a.Index != REG_TLS {
+	if a.Index != REG_NONE && a.Index != REG_TLS && !(REG_CS <= a.Index && a.Index <= REG_GS) {
 		base := int(a.Reg)
 		switch a.Name {
 		case obj.NAME_EXTERN,
@@ -3724,7 +3724,8 @@ func (ab *AsmBuf) asmandsz(ctxt *obj.Link, cursym *obj.LSym, p *obj.Prog, a *obj
 	}
 
 	if REG_AX <= base && base <= REG_R15 {
-		if a.Index == REG_TLS && !ctxt.Flag_shared && !isAndroid {
+		if a.Index == REG_TLS && !ctxt.Flag_shared && !isAndroid &&
+			!(ctxt.Headtype == objabi.Hwindows && ctxt.Arch.Family == sys.AMD64) {
 			rel = obj.Reloc{}
 			rel.Type = objabi.R_TLS_LE
 			rel.Siz = 4
@@ -5205,21 +5206,6 @@ func (ab *AsmBuf) doasm(ctxt *obj.Link, cursym *obj.LSym, p *obj.Prog) {
 						ab.Put2(0x64, // FS
 							0x8B)
 						ab.asmand(ctxt, cursym, p, &pp.From, &p.To)
-
-					case objabi.Hwindows:
-						// Windows TLS base is always 0x28(GS).
-						pp.From = p.From
-
-						pp.From.Type = obj.TYPE_MEM
-						pp.From.Name = obj.NAME_NONE
-						pp.From.Reg = REG_GS
-						pp.From.Offset = 0x28
-						pp.From.Index = REG_NONE
-						pp.From.Scale = 0
-						ab.rexflag |= Pw
-						ab.Put2(0x65, // GS
-							0x8B)
-						ab.asmand(ctxt, cursym, p, &pp.From, &p.To)
 					}
 				}
 				return

src/cmd/internal/obj/x86/obj6.go

@@ -158,16 +158,33 @@ func progedit(ctxt *obj.Link, p *obj.Prog, newprog obj.ProgAlloc) {
 		}
 	}
 
-	// Android uses a tls offset determined at runtime. Rewrite
+	// Android and Win64 use a tls offset determined at runtime. Rewrite
 	//	MOVQ TLS, BX
 	// to
 	//	MOVQ runtime.tls_g(SB), BX
-	if isAndroid && (p.As == AMOVQ || p.As == AMOVL) && p.From.Type == obj.TYPE_REG && p.From.Reg == REG_TLS && p.To.Type == obj.TYPE_REG && REG_AX <= p.To.Reg && p.To.Reg <= REG_R15 {
+	if (isAndroid || (ctxt.Headtype == objabi.Hwindows && ctxt.Arch.Family == sys.AMD64)) &&
+		(p.As == AMOVQ || p.As == AMOVL) && p.From.Type == obj.TYPE_REG && p.From.Reg == REG_TLS && p.To.Type == obj.TYPE_REG && REG_AX <= p.To.Reg && p.To.Reg <= REG_R15 {
 		p.From.Type = obj.TYPE_MEM
 		p.From.Name = obj.NAME_EXTERN
 		p.From.Reg = REG_NONE
 		p.From.Sym = ctxt.Lookup("runtime.tls_g")
 		p.From.Index = REG_NONE
+		if ctxt.Headtype == objabi.Hwindows {
+			// Win64 requires an additional indirection
+			// to retrieve the TLS pointer,
+			// as runtime.tls_g contains the TLS offset from GS.
+			// add
+			//	MOVQ 0(BX)(GS*1), BX
+			q := obj.Appendp(p, newprog)
+			q.As = p.As
+			q.From = obj.Addr{}
+			q.From.Type = obj.TYPE_MEM
+			q.From.Reg = p.To.Reg
+			q.From.Index = REG_GS
+			q.From.Scale = 1
+			q.From.Offset = 0
+			q.To = p.To
+		}
 	}
 
 	// TODO: Remove.

src/runtime/asm_amd64.s

@@ -201,16 +201,16 @@ nocpuinfo:
 	JZ	needtls
 	// arg 1: g0, already in DI
 	MOVQ	$setg_gcc<>(SB), SI // arg 2: setg_gcc
+	MOVQ	$0, DX	// arg 3, 4: not used when using platform's TLS
+	MOVQ	$0, CX
 #ifdef GOOS_android
 	MOVQ	$runtime·tls_g(SB), DX 	// arg 3: &tls_g
 	// arg 4: TLS base, stored in slot 0 (Android's TLS_SLOT_SELF).
 	// Compensate for tls_g (+16).
 	MOVQ	-16(TLS), CX
-#else
-	MOVQ	$0, DX	// arg 3, 4: not used when using platform's TLS
-	MOVQ	$0, CX
 #endif
 #ifdef GOOS_windows
+	MOVQ	$runtime·tls_g(SB), DX 	// arg 3: &tls_g
 	// Adjust for the Win64 calling convention.
 	MOVQ	CX, R9 // arg 4
 	MOVQ	DX, R8 // arg 3
@@ -251,6 +251,10 @@ needtls:
 	JMP ok
 #endif
 
+#ifdef GOOS_windows
+	CALL	runtime·wintls(SB)
+#endif
+
 	LEAQ	runtime·m0+m_tls(SB), DI
 	CALL	runtime·settls(SB)
 
@@ -2026,6 +2030,9 @@ TEXT runtime·panicSliceConvert<ABIInternal>(SB),NOSPLIT,$0-16
 DATA runtime·tls_g+0(SB)/8, $16
 GLOBL runtime·tls_g+0(SB), NOPTR, $8
 #endif
+#ifdef GOOS_windows
+GLOBL runtime·tls_g+0(SB), NOPTR, $8
+#endif
 
 // The compiler and assembler's -spectre=ret mode rewrites
 // all indirect CALL AX / JMP AX instructions to be

src/runtime/cgo/gcc_windows_amd64.c

@@ -13,11 +13,13 @@
 
 static void threadentry(void*);
 static void (*setg_gcc)(void*);
+static DWORD *tls_g;
 
 void
 x_cgo_init(G *g, void (*setg)(void*), void **tlsg, void **tlsbase)
 {
 	setg_gcc = setg;
+	tls_g = (DWORD *)tlsg;
 }
 
 
@@ -41,8 +43,8 @@ threadentry(void *v)
 	 * Set specific keys in thread local storage.
 	 */
 	asm volatile (
-	  "movq %0, %%gs:0x28\n"	// MOVL tls0, 0x28(GS)
+	  "movq %0, %%gs:0(%1)\n"	// MOVL tls0, 0(tls_g)(GS)
-	  :: "r"(ts.tls)
+	  :: "r"(ts.tls), "r"(*tls_g)
 	);
 
 	crosscall_amd64(ts.fn, setg_gcc, (void*)ts.g);

src/runtime/sys_windows_amd64.s

@@ -8,6 +8,9 @@
 #include "time_windows.h"
 #include "cgo/abi_amd64.h"
 
+// Offsets into Thread Environment Block (pointer in GS)
+#define TEB_TlsSlots 0x1480
+
 // void runtime·asmstdcall(void *c);
 TEXT runtime·asmstdcall(SB),NOSPLIT|NOFRAME,$0
 	// asmcgocall will put first argument into CX.
@@ -303,10 +306,10 @@ TEXT runtime·tstart_stdcall(SB),NOSPLIT,$0
 	MOVQ	AX, g_stackguard1(DX)
 
 	// Set up tls.
-	LEAQ	m_tls(CX), SI
+	LEAQ	m_tls(CX), DI
-	MOVQ	SI, 0x28(GS)
 	MOVQ	CX, g_m(DX)
-	MOVQ	DX, g(SI)
+	MOVQ	DX, g(DI)
+	CALL	runtime·settls(SB) // clobbers CX
 
 	CALL	runtime·stackcheck(SB)	// clobbers AX,CX
 	CALL	runtime·mstart(SB)
@@ -318,7 +321,8 @@ TEXT runtime·tstart_stdcall(SB),NOSPLIT,$0
 
 // set tls base to DI
 TEXT runtime·settls(SB),NOSPLIT,$0
-	MOVQ	DI, 0x28(GS)
+	MOVQ	runtime·tls_g(SB), CX
+	MOVQ	DI, 0(CX)(GS)
 	RET
 
 // Runs on OS stack.
@@ -404,3 +408,32 @@ TEXT runtime·osSetupTLS(SB),NOSPLIT,$0-8
 	LEAQ	m_tls(AX), DI
 	CALL	runtime·settls(SB)
 	RET
+
+// This is called from rt0_go, which runs on the system stack
+// using the initial stack allocated by the OS.
+TEXT runtime·wintls(SB),NOSPLIT|NOFRAME,$0
+	// Allocate a TLS slot to hold g across calls to external code
+	MOVQ	SP, AX
+	ANDQ	$~15, SP	// alignment as per Windows requirement
+	SUBQ	$48, SP	// room for SP and 4 args as per Windows requirement
+			// plus one extra word to keep stack 16 bytes aligned
+	MOVQ	AX, 32(SP)
+	MOVQ	runtime·_TlsAlloc(SB), AX
+	CALL	AX
+	MOVQ	32(SP), SP
+
+	MOVQ	AX, CX	// TLS index
+
+	// Assert that slot is less than 64 so we can use _TEB->TlsSlots
+	CMPQ	CX, $64
+	JB	ok
+	CALL	runtime·abort(SB)
+ok:
+	// Convert the TLS index at CX into
+	// an offset from TEB_TlsSlots.
+	SHLQ	$3, CX
+
+	// Save offset from TLS into tls_g.
+	ADDQ	$TEB_TlsSlots, CX
+	MOVQ	CX, runtime·tls_g(SB)
+	RET