[−][src]Module sunrise_kernel::i386::gdt

GDT Handler

The Global Descriptor Table is responsible for segmentation of memory.

Since we manage memory permissions in the paging, we want to set-up our segments so that we have a flat-memory model, i.e. having segments with base = 0; limit = 0xffffffff.

GDT segments

Index	Found in	Maps to	Purpose
`GdtIndex::Null`	nowhere (hopefully)	_	_
`GdtIndex::KCode`	`cs`, while in kernel code	flat: `0x00000000..0xffffffff`	kernel's code segment
`GdtIndex::KData`	`ds`, `es`, while in kernel code	flat: `0x00000000..0xffffffff`	kernel's data segment
`GdtIndex::KTls`	`gs`, while in kernel code	kernel's cpu-locals	kernel sets-up cpu-locals at this address
`GdtIndex::KStack`	`ss`, while in kernel code	flat: `0x00000000..0xffffffff`	kernel's stack segment
`GdtIndex::UCode`	`cs`, while in user code	flat: `0x00000000..0xffffffff`	user's code segment
`GdtIndex::UData`	`ds`, `es`, while in user code	flat: `0x00000000..0xffffffff`	user's data segment
`GdtIndex::UTlsRegion`	`fs`, while in user code	`&TLS..&TLS+0x200`	user can get the address of its `TLS` from this selector
`GdtIndex::UTlsElf`	`gs`, while in user code	User-defined	user can set-up elf TLS at this address
`GdtIndex::UStack`	`ss`, while in user code	flat: `0x00000000..0xffffffff`
`GdtIndex::LDT`	_	Points to the `GLOBAL_LDT`
`GdtIndex::TSS`	IDT Double fault vector	Points to the `MAIN_TASK`	Double fault exception backups registers to this TSS
`GdtIndex::FTSS`	IDT Double fault vector		Double fault exception loads registers from this TSS

UTlsRegion

The kernel allocates a 0x200-bytes region for every thread, and always makes fs point to it when jumping to userspace. See TLS for more.

This region is thread local, its address is switched at every thread-switch.

The segment pointed by gs is controlled by the user. It can set its address/limit with svcSetThreadArea. The segment it chooses to use is local to every thread, and defaults to 0x00000000..0xffffffff.

Typically, the user will want to make gs point to its elf TLS.

This segment is thread local, its address and size are switched at every thread-switch.

LDT segments:

None :)

x86_64

Because x86_64 uses fs for tls instead of gs, the purpose of gs and fs are swapped:

Index	Found in	Maps to	Purpose
MSR	`fs`, while in kernel code	kernel's cpu-locals	kernel sets-up cpu-locals at this address
MSR	`gs`, while in user code	`&TLS..&TLS+0x200`	user can get the address of its `TLS` from this selector
MSR	`fs`, while in user code	User-defined	user can set-up elf TLS at this address

Structs

DescriptorTable	A structure containing our GDT.
DescriptorTableEntry	An entry in the GDT/LDT.
DoubleFaultTaskStack	The stack used while handling a double fault.
GdtManager	Safety wrapper that manages the lifetime of GDT tables.
MainTask	The main TSS. See MAIN_TASK.

Enums

GdtIndex	Index in the GDT of each segment descriptor.
SystemDescriptorTypes	Lists the valid values of System Descriptor Types.

Statics

DOUBLE_FAULT_TASK	Double fault TSS
DOUBLE_FAULT_TASK_STACK	The stack used while handling a double fault. See DOUBLE_FAULT_TASK.
GDT	The global GDT. Needs to be initialized with init_gdt.
GLOBAL_LDT	The global LDT used by all the processes.
MAIN_TASK	Main TSS

Functions

init_gdt

Initializes the GDT.