Operating Systems (Fall/Winter 2018) Linux Paging Yajin Zhou (http://yajin.org) Zhejiang University Acknowledgement: some pages are based on the slides from Zhi Wang(fsu). Introduction X86 Registers • General purpose registers • Segment registers • Control registers • EIP (PC) • EFLAGS System Registers GDTR, LDTR • GDTR: GDT base address and limit • Lgdt sgdt: instructions to load/store GDTR • LGTR Address Translation Step I: logical address to linear address • 根据指令的性质确定使⽤哪⼀个 段寄存器 • 根据段寄存器内容，找到 segment descriptor • 从哪⾥找？GDT/LDT，表的地 址在GDTR,LDTR • 根据segment descriptor找到base address。然后⽐较limit，和检查 访问权限 • liner address = base + offset Step I: logical address to linear address • Q1: how to get segment selector? • DS,CS and etc • Q2: how to get the address of descriptor table? • GDTR, LDTR Step I: logical address to linear address Segment descriptor Step I: logical address to linear address Step II: liner address to physical address • CR3: get the base address of page directory • find the base address of page table based on [31:22] of linear address • Find the page table entry • Get the physical address CR3 and PDE/PTE • CR3 is in PCB and saved/restored during context switch PAE • Physical address extension • 4G physical address space -> 64GB • 32 bit linear address ->52 bit physical address CR3 in PAE • 31:5 -> Page directory pointer • 4 entries, each entry 64 bit. PDPTE0, PDPTE1, PDPTE2, PDPTE3 PDPTE Linear to physical address with PAE • Step I: select PDPTE using [31:30] of linear address. Bit p of PDPTE should be 1 • Step II: address of page directory table is in [51:12] of PDPTE (相当于 CR3)。 There are 512 entries (8 bytes each) in page directory table. Each entry is called page directory entry (PDE). • PDE: [51:12] : [51:12] of PDPTE . [11:3] : [29:21] of linear address, bit [2:0] is zero • Step III: find PTE (page table entry) • PTE: bit [51:12]: PDE. [11:3]: [20:12] of linear address, [2:0]: 0 • Step IV: get physical address. [51:12]: PTE, BIT [11:0]: liner address CR3 Talk is cheap, show me the code! Setup Segment Registers • Set up segment registers when creating a new thread Value • __KERNEL_CS: 12X8 = 96 • __KERNEL_DS: 13X8 = 104 • __USER_DS = 15X8 + 3 = 120 + 3 • __USER_CS = 14X8 + 3 = 110 + 3 Linux uses GDT, not LDT We will show the value of the GDT entry later! Context switch Experiment Ubuntu 1004, 32 bit 参考: http://ilinuxkernel.com/?p=1276 Kernel module • Dump registers Kernel module • Read physical address from a character device Tool to dump memory in user space : Results Output of the user program • PAE is enabled. • GDTR is at 0xc1a40000 (linear address) -> physical address: 0x1a40000. • Temp address: 0xBF95216C Logical -> Linear • GDT entry at index 15 (offset is 15 x 8). • 0x00cff3000000FFFF Logical -> Linear • 0x00cff3000000FFFF -> Base address = 0 • Linear address = logical address Segment descriptor Linear -> Physical • Temp address: BF95216C • 1011 1111 1001 0101 0010 0001 0110 1100 • Locate PDPTE • cr3=2E1DF000 • PDPTE2=0x000000002E2D2001 -> page directory base address (physical address) = 0x2e2d2000 Linear -> Physical • 0x2e2d2000 + 0x1fc(11 1111 100)x 8 = 0x2e2d2FE0 -> page directory entry address • Page table base address: 0x00002E4E4000 (67 is flag) Linear -> Physical • PTE address: 0x00002E4E4000 + 1 0101 0010b*8 = 0x00002E4E4a90 • Physical frame address: 0x2649900 (0x67 and the highest 8: flag) • Physical address: 0x26499000 + 0001 0110 1100b = 0X2649916c • 0x2649916c Logical Address Linear Address Physical Address • 0xBF95216C -> 0xBF95216C -> 0x2649916c HW9 is out.