kernel/amd64: Add task state segment logic

This commit implements the task state segment and splits up processor
initialization into two seperate stages. The cpu_conf() function is
apart of the first stage and sets up things that should be going by the
time the kernel is started / early init. The cpu_init() function runs
later functions that initialize further platform specific subsystems.

Signed-off-by: Ian Moffett <ian@osmora.org>

3 files changed, 162 insertions, 0 deletions

diff --git a/src/sys/include/arch/amd64/mdcpu.h b/src/sys/include/arch/amd64/mdcpu.h
index 0a39eb4..1503adb 100644
--- a/src/sys/include/arch/amd64/mdcpu.h
+++ b/src/sys/include/arch/amd64/mdcpu.h
@@ -32,6 +32,7 @@
 
 #include <sys/types.h>
 #include <sys/cdefs.h>
+#include <machine/tss.h>
 
 #define md_spinwait() __ASMV("pause")
 #define md_intoff()   __ASMV("cli")
@@ -46,12 +47,14 @@
  * @cr3: CR3 register value (PML<n> phys)
  * @lapic_base: LAPIC register interface base
  * @x2apic: Has the x2APIC? Is 1 if true
+ * @tss: Task state segment for this core
  */
 struct mdcore {
     uint32_t apic_id;
     uint64_t cr3;
     void *lapic_base;
     uint8_t x2apic : 1;
+    struct tss_entry tss;
 };
 
 #endif  /* !_MACHINE_MDCPU_H_ */
diff --git a/src/sys/include/arch/amd64/tss.h b/src/sys/include/arch/amd64/tss.h
new file mode 100644
index 0000000..da2059c
--- /dev/null
+++ b/src/sys/include/arch/amd64/tss.h
@@ -0,0 +1,149 @@
+/*
+ * Copyright (c) 2025 Ian Marco Moffett and L5 engineers
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of the project nor the names of its
+ *    contributors may be used to endorse or promote products derived from
+ *    this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef _MACHINE_TSS_H_
+#define _MACHINE_TSS_H_ 1
+
+#include <sys/types.h>
+#include <sys/cdefs.h>
+
+#define IST_SCHED   1U
+
+struct pcore;
+
+/*
+ * A TSS entry (64-bit)
+ *
+ * See Intel SDM Section 8.2.1 - Task-State Segment (TSS)
+ */
+struct __packed tss_entry {
+    uint32_t reserved1;
+    uint32_t rsp0_lo;
+    uint32_t rsp0_hi;
+    uint32_t rsp1_lo;
+    uint32_t rsp1_hi;
+    uint32_t rsp2_lo;
+    uint32_t rsp2_hi;
+    uint64_t reserved2;
+    uint32_t ist1_lo;
+    uint32_t ist1_hi;
+    uint32_t ist2_lo;
+    uint32_t ist2_hi;
+    uint32_t ist3_lo;
+    uint32_t ist3_hi;
+    uint32_t ist4_lo;
+    uint32_t ist4_hi;
+    uint32_t ist5_lo;
+    uint32_t ist5_hi;
+    uint32_t ist6_lo;
+    uint32_t ist6_hi;
+    uint32_t ist7_lo;
+    uint32_t ist7_hi;
+    uint64_t reserved3;
+    uint16_t reserved4;
+    uint16_t io_base;
+    uint8_t iomap[8192];
+};
+
+/*
+ * TSS descriptor (64-bit)
+ *
+ * The TSS descriptor describes the location
+ * of the TSS segments among other things...
+ *
+ * See Intel SDM Section 8.2.3 - TSS Descriptor in 64-bit mode
+ */
+struct __packed tss_desc {
+    uint16_t seglimit;
+    uint16_t base_lo16;
+    uint8_t base_mid8;
+    uint8_t type        : 4;
+    uint8_t zero        : 1;
+    uint8_t dpl         : 2;
+    uint8_t p           : 1;
+    uint8_t seglimit_hi : 4;
+    uint8_t avl         : 1;
+    uint8_t unused      : 2;
+    uint8_t g           : 1;
+    uint8_t base_hi_mid8;
+    uint32_t base_hi32;
+    uint32_t reserved;
+};
+
+/*
+ * Holds the address of the address pointing
+ * to the top of an interrupt stack.
+ */
+union tss_stack {
+    struct {
+        uint32_t top_lo;
+        uint32_t top_hi;
+    };
+    uint64_t top;
+};
+
+__always_inline static inline void
+tss_load(void)
+{
+    __ASMV("str %ax\n"
+           "mov $0x2B, %ax\n"
+           "ltr %ax"
+    );
+}
+
+/*
+ * Allocates TSS stack.
+ *
+ * @entry_out: Result is written here
+ * @size: Size of new stack
+ *
+ * Returns 0 on success.
+ */
+int tss_alloc_stack(union tss_stack *entry_out, size_t size);
+
+/*
+ * Update interrupt stack table entry `istno' with `stack'
+ *
+ * @pcore: Current processor
+ * @stack: Interrupt stack.
+ * @istno: IST number, must be 1-based.
+ *
+ * Returns 0 on success.
+ */
+int tss_update_ist(struct pcore *pcore, union tss_stack stack, uint8_t istno);
+
+/*
+ * Write the TSS with a known state
+ *
+ * @pcore: Current processor
+ * @desc: Descriptor to initialize
+ */
+void write_tss(struct pcore *pcore, struct tss_desc *desc);
+
+#endif  /* !_MACHINE_TSS_H_ */
diff --git a/src/sys/include/sys/cpuvar.h b/src/sys/include/sys/cpuvar.h
index b5ff31b..7431ce4 100644
--- a/src/sys/include/sys/cpuvar.h
+++ b/src/sys/include/sys/cpuvar.h
@@ -63,6 +63,16 @@ struct pcore {
 void cpu_conf(struct pcore *pcore);
 
 /*
+ * Initialize a processor core on the system, second
+ * stage initialization hook.
+ *
+ * [MD]
+ *
+ * @pcore: Processor core to init
+ */
+void cpu_init(struct pcore *pcore);
+
+/*
  * Get the current processing element (core) as
  * a 'pcore' descriptor.
  *