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Alexander Kuleshov
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KernelStructures/README.md
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# Internal `system` structures of the Linux kernel
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This is not usual chapter of `linux-insides`. As you may understand from the title, it mostly describes
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internal `system` structures of the Linux kernel. Like `Interrupt Descriptor Table`, `Global Descriptor
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Table` and many many more.
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Most of information is taken from official [Intel](http://www.intel.com/content/www/us/en/processors/architectures-software-developer-manuals.html) and [AMD](http://developer.amd.com/resources/developer-guides-manuals/) manuals.
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KernelStructures/idt.md
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interrupt-descriptor table (IDT)
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================================================================================
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Three general interrupt & exceptions sources:
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* Exceptions - sync;
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* Software interrupts - sync;
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* External interrupts - async.
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Types of Exceptions:
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* Faults - are precise exceptions reported on the boundary `before` the instruction causing the exception. The saved `%rip` points to the faulting instruction;
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* Traps - are precise exceptions reported on the boundary `following` the instruction causing the exception. The same with `%rip`;
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* Aborts - are imprecise exceptions. Because they are imprecise, aborts typically do not allow reliable program restart.
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`Maskable` interrupts trigger the interrupt-handling mechanism only when RFLAGS.IF=1. Otherwise they are held pending for as long as the RFLAGS.IF bit is cleared to 0.
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`Nonmaskable` interrupts (NMI) are unaffected by the value of the rFLAGS.IF bit. However, the occurrence of an NMI masks further NMIs until an IRET instruction is executed.
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Specific exception and interrupt sources are assigned a fixed vector-identification number (also called an “interrupt vector” or simply “vector”). The interrupt vector is used by the interrupt-handling mechanism to locate the system-software service routine assigned to the exception or interrupt. Up to
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256 unique interrupt vectors are available. The first 32 vectors are reserved for predefined exception and interrupt conditions. They are defined in the [arch/x86/include/asm/traps.h](http://lxr.free-electrons.com/source/arch/x86/include/asm/traps.h#L121) header file:
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```
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/* Interrupts/Exceptions */
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enum {
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X86_TRAP_DE = 0, /* 0, Divide-by-zero */
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X86_TRAP_DB, /* 1, Debug */
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X86_TRAP_NMI, /* 2, Non-maskable Interrupt */
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X86_TRAP_BP, /* 3, Breakpoint */
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X86_TRAP_OF, /* 4, Overflow */
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X86_TRAP_BR, /* 5, Bound Range Exceeded */
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X86_TRAP_UD, /* 6, Invalid Opcode */
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X86_TRAP_NM, /* 7, Device Not Available */
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X86_TRAP_DF, /* 8, Double Fault */
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X86_TRAP_OLD_MF, /* 9, Coprocessor Segment Overrun */
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X86_TRAP_TS, /* 10, Invalid TSS */
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X86_TRAP_NP, /* 11, Segment Not Present */
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X86_TRAP_SS, /* 12, Stack Segment Fault */
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X86_TRAP_GP, /* 13, General Protection Fault */
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X86_TRAP_PF, /* 14, Page Fault */
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X86_TRAP_SPURIOUS, /* 15, Spurious Interrupt */
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X86_TRAP_MF, /* 16, x87 Floating-Point Exception */
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X86_TRAP_AC, /* 17, Alignment Check */
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X86_TRAP_MC, /* 18, Machine Check */
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X86_TRAP_XF, /* 19, SIMD Floating-Point Exception */
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X86_TRAP_IRET = 32, /* 32, IRET Exception */
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};
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```
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Error Codes
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--------------------------------------------------------------------------------
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The processor exception-handling mechanism reports error and status information for some exceptions using an error code. The error code is pushed onto the stack by the exception-mechanism during the control transfer into the exception handler. The error code has two formats:
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* most error-reporting exceptions format;
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* page fault format.
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Here is format of selector error code:
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```
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31 16 15 3 2 1 0
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+-------------------------------------------------------------------------------+
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| | | T | I | E |
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| Reserved | Selector Index | - | D | X |
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| | | I | T | T |
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+-------------------------------------------------------------------------------+
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```
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Where:
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* `EXT` - If this bit is set to 1, the exception source is external to the processor. If cleared to 0, the exception source is internal to the processor;
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* `IDT` - If this bit is set to 1, the error-code selector-index field references a gate descriptor located in the `interrupt-descriptor table`. If cleared to 0, the selector-index field references a descriptor in either the `global-descriptor table` or local-descriptor table `LDT`, as indicated by the `TI` bit;
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* `TI` - If this bit is set to 1, the error-code selector-index field references a descriptor in the `LDT`. If cleared to 0, the selector-index field references a descriptor in the `GDT`.
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* `Selector Index` - The selector-index field specifies the index into either the `GDT`, `LDT`, or `IDT`, as specified by the `IDT` and `TI` bits.
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Page-Fault Error Code format is:
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```
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31 4 3 2 1 0
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+-------------------------------------------------------------------------------+
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| | | R | U | R | - |
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| Reserved | I/D | S | - | - | P |
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| | | V | S | W | - |
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+-------------------------------------------------------------------------------+
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```
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Where:
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* `I/D` - If this bit is set to 1, it indicates that the access that caused the page fault was an instruction fetch;
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* `RSV` - If this bit is set to 1, the page fault is a result of the processor reading a 1 from a reserved field within a page-translation-table entry;
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* `U/S` - If this bit is cleared to 0, an access in supervisor mode (`CPL=0, 1, or 2`) caused the page fault. If this bit is set to 1, an access in user mode (CPL=3) caused the page fault;
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* `R/W` - If this bit is cleared to 0, the access that caused the page fault is a memory read. If this bit is set to 1, the memory access that caused the page fault was a write;
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* `P` - If this bit is cleared to 0, the page fault was caused by a not-present page. If this bit is set to 1, the page fault was caused by a page-protection violation.
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Interrupt Control Transfers
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--------------------------------------------------------------------------------
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The IDT may contain any of three kinds of gate descriptors:
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* `Task Gate` - contains the segment selector for a TSS for an exception and/or interrupt handler task;
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* `Interrupt Gate` - contains segment selector and offset that the processor uses to transfer program execution to a handler procedure in an interrupt handler code segment;
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* `Trap Gate` - contains segment selector and offset that the processor uses to transfer program execution to a handler procedure in an exception handler code segment.
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General format of gates is:
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```
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127 96
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+-------------------------------------------------------------------------------+
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| |
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| Reserved |
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| |
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+--------------------------------------------------------------------------------
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95 64
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+-------------------------------------------------------------------------------+
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| |
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| Offset 63..32 |
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| |
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+-------------------------------------------------------------------------------+
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63 48 47 46 44 42 39 34 32
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+-------------------------------------------------------------------------------+
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| | | D | | | | | | |
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| Offset 31..16 | P | P | 0 |Type |0 0 0 | 0 | 0 | IST |
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| | | L | | | | | | |
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-------------------------------------------------------------------------------+
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31 16 15 0
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+-------------------------------------------------------------------------------+
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| | |
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| Segment Selector | Offset 15..0 |
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| | |
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+-------------------------------------------------------------------------------+
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```
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Where
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* `Selector` - Segment Selector for destination code segment;
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* `Offset` - Offset to handler procedure entry point;
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* `DPL` - Descriptor Privilege Level;
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* `P` - Segment Present flag;
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* `IST` - Interrupt Stack Table;
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* `TYPE` - one of: Local descriptor-table (LDT) segment descriptor, Task-state segment (TSS) descriptor, Call-gate descriptor, Interrupt-gate descriptor, Trap-gate descriptor or Task-gate descriptor.
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An `IDT` descriptor is represented by the following structure in the Linux kernel (only for `x86_64`):
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```C
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struct gate_struct64 {
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u16 offset_low;
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u16 segment;
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unsigned ist : 3, zero0 : 5, type : 5, dpl : 2, p : 1;
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u16 offset_middle;
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u32 offset_high;
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u32 zero1;
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} __attribute__((packed));
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```
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which is defined in the [arch/x86/include/asm/desc_defs.h](http://lxr.free-electrons.com/source/arch/x86/include/asm/desc_defs.h#L51) header file.
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A task gate descriptor does not contain `IST` field and its format differs from interrupt/trap gates:
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```C
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struct ldttss_desc64 {
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u16 limit0;
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u16 base0;
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unsigned base1 : 8, type : 5, dpl : 2, p : 1;
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unsigned limit1 : 4, zero0 : 3, g : 1, base2 : 8;
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u32 base3;
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u32 zero1;
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} __attribute__((packed));
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```
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Exceptions During a Task Switch
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--------------------------------------------------------------------------------
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An exception can occur during a task switch while loading a segment selector. Page faults can also occur when accessing a TSS. In these cases, the hardware task-switch mechanism completes loading the new task state from the TSS, and then triggers the appropriate exception mechanism.
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**In long mode, an exception cannot occur during a task switch, because the hardware task-switch mechanism is disabled.**
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Nonmaskable interrupt
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--------------------------------------------------------------------------------
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**TODO**
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API
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--------------------------------------------------------------------------------
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**TODO**
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Interrupt Stack Table
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--------------------------------------------------------------------------------
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**TODO**
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@ -77,5 +77,7 @@
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* [Linux kernel development](Misc/contribute.md)
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* [Write and Submit your first Linux kernel Patch]()
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* [Data types in the kernel]()
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* [KernelStructures](KernelStructures/README.md)
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* [IDT](KernelStructures/idt.md)
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* [Useful links](LINKS.md)
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* [Contributors](contributors.md)
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