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3.6. SWEB TASK SEPARATION AND PROTECTION CHAPTER 3. VM, PROTECTION AND PAGING 3.6 Sweb Task Separation and Protection 3.6.1 Segment Descriptor Privilege Level / Task Privilege Level As shown in Table 3.8, Sweb uses only privilege level 0 for kernel code and 3 for user code. User threads running user code start with CS and DS set to 4 and 5 respectively. Refer to low level task switching and syscalls on when privilege levels are changed. 3.6.2 User- / Supervisor- Page Paging offers an additional mechanism concerning code privileges in form of the user/supervisoraccess flag in the Page-Directory. Pages where the user_access flag is not set may only be accessed by code with privilege level 0 i.e. the kernel. In Sweb the user_access flag is set only on mappings below 2GiB. 3.6.3 LoadPageOnDemand Instead of loading a new process complete into memory on process launch, only the pages needed can be loaded one a time. In Sweb, on start of an new user process, all virtual pages are non-present. The first time each user page is accessed, a page fault happens and the Page-Fault-Handler is called. On deciding that the page is in the user threads linear address space below 2GiB, the user processes instance of class Loader is called. Loader then checks the ELF-Header and loads the code and data from the executable, that match the missing page’s linear address range. It is further possible to remember if a page has never changed since it was loaded on demand. Then, instead of swapping it out, it could simply be free’d because it can be reload from the executable. This approach however introduces some issues in conjunction with shared pages which are detailed in section 3.7.6. 3.6.4 UserThread Termination On termination of a user process, ArchMemory::freePageDirectory(..) is called. The function recursively frees all mapped pages below 2GiB. 3.7 Extending Sweb 3.7.1 Using 4MiB Pages In various operating systems, pages of 4MiB size are used to map kernel code because the IA32 maintains different TLBs for 4KiB and 4MiB pages. The advantage would be that the kernel has its own page cache that would remain unchanged on a task switch. In the beginning Sweb used one 4MiB kernel page. Later, it was more important to prevent and catch errors by marking parts (
CHAPTER 3. VM, PROTECTION AND PAGING 3.7. EXTENDING SWEB ArchMemory functions always take page numbers which depend on the given page size as arguments, i.e. the 2nd 4MiB Page and the 2048th 4KiB Page would start at the same linear address. In order to map 4MiB pages in Sweb during runtime, the PageManager needs to be modified. Since the PageManager manages 4MiB pages as 1024 consecutive 4KiB pages, it needs to be able to find 1024 consecutive free pages starting from a page number that is a multiple of 1024. When a free 4MiB page has been found, ArchMemory::map( , , , , PAGE_SIZE*1024); can be used to create a mapping. Unmapping is already supported. 3.7.2 Additional pages for the kernel Currently kernel memory is limited to 4MiB. Even though dynamically mapping pages beyond the 4MiB area of the kernel is possible, it is easier to make available more memory at at boot time by changing the file init_boottime_pagetables.cpp. To do this, Page-Directory-Entry 513 (from 2GiB+4MiB to 2GiB+8MiB) needs to be present and point to a matching Page-Table which can then map up to 4MiB more memory. Since PageManger detects kernel pages auto-magically at initialisation time, all that remains is to modify the pointer end_address in main.cpp. Note however that 4MiB are often enough. The Kernel code itself uses about 500KiB. PageManager usually needs less than 10KiB with 32MiB RAM and KMM uses about 4KiB per allocation. Additionally FiFos and RingBuffers use about 50KiB buffer space. The conclusion is, that running out of kernel memory in Sweb is more likely the result of a bug than shortage. 3.7.3 sharing Memory between linear address spaces From what the reader of this document now knows about Paging, it is clear that sharing memory between different linear address spaces (and their programs) means sharing pages. Sweb does this already, kernel pages are shared shared between all user processes because their mapping exits in all page directories. So all that needs to be done is to map one linear memory page of different user processes to the same physical page. Finally, the global_page flag of the mapping needs to be set, in order to distinguish it from non-shared mappings and instruct the CPU to keep the mapping cached on a task switch. In order for the CPU to honour the global flag, the PGE flag in CR4 needs to be set. (see table 3.4) Furthermore mutual exclusion between user programs that share pages must not be forgotten. Care must be taken when a user processes with shared pages terminates. The PageManager must be changed, so that pages are marked free only if all owning processes have free’d it. 3.7.4 Cloning a Tasks linear address space Syscalls like clone() or fork() require the creation of a new user process which’s linear address space is identical to an existing user process. Unless we elegantly use Copy- OnWrite, this means manually copying every page to a new physical memory location and creating a new Page-Directory that maps to the new locations. This could be done from the old user threads linear memory context by referencing the new physical locations through the identity mapping above 3GiB. 47 of 151
Page 1 and 2: The holy bible of SWEB SWEB-Crew Se
Page 3 and 4: Contents 1 Install 13 1.1 some note
Page 5 and 6: CONTENTS CONTENTS 6 System Calls 71
Page 7 and 8: CONTENTS CONTENTS 11.6.9Running Xen
Page 9 and 10: List of Figures 3.1 Protected Mode
Page 11 and 12: List of Tables 3.1 Segment Descript
Page 13 and 14: Chapter 1 Install Basic guidelines
Page 15 and 16: CHAPTER 1. INSTALL 1.2. RUNNING SWE
Page 17 and 18: CHAPTER 1. INSTALL 1.3. DEVELOPERS
Page 19 and 20: Chapter 2 GDB 2.1 General The GNU P
Page 21 and 22: CHAPTER 2. GDB 2.5. USING DDD The d
Page 23 and 24: CHAPTER 2. GDB 2.5. USING DDD To ge
Page 25 and 26: Chapter 3 VM, Protection and Paging
Page 27 and 28: CHAPTER 3. VM, PROTECTION AND PAGIN
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Page 55 and 56: Chapter 4 Kernelspace Memory Manage
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Chapter 8 Block Devices 8.1 Introdu
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Chapter 9 VFS, FS 9.1 Introduction
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CHAPTER 9. VFS, FS 9.3. VFS DATA ST
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CHAPTER 9. VFS, FS 9.4. RAMFS - MEM
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Chapter 10 Locking 10.1 Introductio
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CHAPTER 10. LOCKING 10.2. LOCKS 10.
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CHAPTER 10. LOCKING 10.4. CRITICAL
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Chapter 11 Xen 11.1 Introduction Th
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CHAPTER 11. XEN 11.3. XEN Xen diffe
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CHAPTER 11. XEN 11.4. SWEB- XEN 11.
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CHAPTER 11. XEN 11.6. APPENDIX: INS
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Chapter 12 Hacking Sweb This chapte
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Bibliography [1] Wikipedia:Binary_p
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