From c3e5fde3a2df05f7ba1b23e3e38b199bb6ae71e5 Mon Sep 17 00:00:00 2001
From: tkchia <tkchia-cosmo@gmx.com>
Date: Sat, 24 Sep 2022 07:24:02 +0000
Subject: [PATCH] Bare metal: reserve 2 GDT slots for TSS

---
 ape/ape.S                  | 25 ++++++++++++++++++-------
 ape/ape.lds                |  3 +++
 ape/macros.internal.h      | 23 +++++++++++++++++++++++
 libc/runtime/pc.internal.h |  1 +
 4 files changed, 45 insertions(+), 7 deletions(-)

diff --git a/ape/ape.S b/ape/ape.S
index aada551aa..7728a9fc2 100644
--- a/ape/ape.S
+++ b/ape/ape.S
@@ -1137,12 +1137,17 @@ sconf:	.short	1843200/*hz*/ / 16/*wut*/ / 9600/*baud*/
 	.endobj	sconf,global,hidden
 
 //	Global Descriptor Table
-//
-//	@note address portion only concern legacy modes
 	.align	8
-gdt:	.short	2f-1f		# table byte length
-	.long	REAL(1f),0	# table address
-	.zero	2
+_gdtrphy:
+	.short	2f-1f-1		# table byte length
+	.long	REAL(1f)	# table address (physical space)
+	.endobj	_gdtrphy,global,hidden
+_gdtr:
+	.short	2f-1f-1		# table byte length
+	.quad	1f		# table address (final virtual space)
+	.endobj	_gdtr,global,hidden
+	.align	8
+_gdt:
 1:
 //	          ┌G:granularity (1 → limit *= 0x1000)
 //	          │┌D/B:default operation size (0 = 16|64bit, 1 = 32-bit)
@@ -1172,7 +1177,9 @@ gdt:	.short	2f-1f		# table byte length
 .quad	0b0000000011001111100100100000000000000000000000001111111111111111 #32
 .quad	0b0000000010101111100110110000000000000000000000001111111111111111 #40
 .quad	0b0000000010101111100100110000000000000000000000001111111111111111 #48
-2:	.endobj	gdt,global,hidden
+.tssdescstub _tss							#56,64
+2:
+	.endobj	_gdt,global,hidden
 
 /*─────────────────────────────────────────────────────────────────────────────╗
 │ αcτµαlly pδrταblε εxεcµταblε § real mode                                     │
@@ -1433,7 +1440,7 @@ golong:	cli
 	rdmsr
 	or	$EFER_LME|EFER_SCE,%eax
 	wrmsr
-	lgdt	REAL(gdt)
+	lgdt	REAL(_gdtrphy)
 	mov	%cr0,%eax
 	or	$CR0_PE|CR0_PG|CR0_MP,%eax
 	and	$~CR0_EM,%eax
@@ -1451,6 +1458,8 @@ long:	xor	%eax,%eax
 	mov	%eax,%fs
 	mov	%eax,%gs
 	mov	$0x80000,%esp
+	mov	$GDT_LONG_TSS,%al
+	ltr	%ax
 	xor	%r12d,%r12d
 	xor	%r13d,%r13d
 	xor	%r14d,%r14d
@@ -1464,6 +1473,8 @@ long:	xor	%eax,%eax
 	call	__map_phdrs
 	push	$0x037f
 	fldcw	(%rsp)
+	lgdt	_gdtr					# reload GDTR for
+							# virtual memory space
 	movabs	$kernel,%rax
 	jmp	*%rax
 	.endfn	long
diff --git a/ape/ape.lds b/ape/ape.lds
index 05cee5ba1..a756c9340 100644
--- a/ape/ape.lds
+++ b/ape/ape.lds
@@ -572,6 +572,9 @@ HIDDEN(v_ape_realslacksectors =
 HIDDEN(v_ape_realpages = v_ape_realsectors / (4096 / 512));
 HIDDEN(v_ape_highsectors =
            (ROUNDUP(RVA(_edata), 512) / 512) - v_ape_realsectors);
+PROVIDE_HIDDEN(_tss = 0);
+PROVIDE_HIDDEN(_tss_end = 0);
+TSSDESCSTUB2(_tss, _tss, _tss_end ? _tss_end - _tss - 1 : 0);
 #endif
 
 /* ZIP End of Central Directory header */
diff --git a/ape/macros.internal.h b/ape/macros.internal.h
index c17344116..7ba52bafa 100644
--- a/ape/macros.internal.h
+++ b/ape/macros.internal.h
@@ -180,6 +180,12 @@
  .stub	\name\()_bcs\n,long
 .endm
 
+//	Task State Segment Descriptor Entries.
+.macro	.tssdescstub name:req
+ .stub	\name\()_desc_ent0,quad
+ .stub	\name\()_desc_ent1,quad
+.endm
+
 /* clang-format on */
 #elif defined(__LINKER__)
 
@@ -257,5 +263,22 @@
                        (X) % 10 * 0x1000000        \
                      : 0xffffffffffffffff)
 
+/**
+ * Laying out the GDT entries for a TSS for bare metal operation.
+ */
+#define TSSDESCSTUB2(SYM, BASE, LIM)                 \
+  HIDDEN(SYM##_desc_ent0 = TSSDESC_ENT0(BASE, LIM)); \
+  HIDDEN(SYM##_desc_ent1 = TSSDESC_ENT1(BASE));      \
+  ASSERT((LIM) >= 0 && (LIM) <= 0xffff, "bare metal TSS is suspiciously fat")
+#define TSSDESC_ENT0(BASE, LIM)                \
+  (((LIM)        <<  0 & 0x000000000000ffff) | \
+   ((BASE)       << 16 & 0x000000ffffff0000) | \
+    0x89         << 40                       | \
+   ((LIM)  >> 16 << 48 & 0x000f000000000000) | \
+    0x2          << 52                       | \
+   ((BASE) >> 24 << 56 & 0xff00000000000000))
+#define TSSDESC_ENT1(BASE)                     \
+   ((BASE) >> 32 <<  0 & 0x00000000ffffffff)
+
 #endif /* __ASSEMBLER__ */
 #endif /* APE_MACROS_H_ */
diff --git a/libc/runtime/pc.internal.h b/libc/runtime/pc.internal.h
index 45ca8d919..6ff21bfdf 100644
--- a/libc/runtime/pc.internal.h
+++ b/libc/runtime/pc.internal.h
@@ -120,6 +120,7 @@
 #define GDT_LEGACY_DATA 32
 #define GDT_LONG_CODE   40
 #define GDT_LONG_DATA   48
+#define GDT_LONG_TSS    56
 
 #define PIC1         0x20 /* IO base address for master PIC */
 #define PIC2         0xA0 /* IO base address for slave PIC */