SC2440 virtual address to physical address conversion

Disclaimer: The conclusions drawn in this article are all from the "Embedded Linux Complete Development Manual". They are summarized in my own words after trying to understand its meaning, so as to better understand the contents of this section.

VA: virtual address 
WVA: converted virtual address 
PA: physical address 
PID: process identification number, obtained by C13 of CP15.

• 1. The virtual address gets the converted virtual address

• If VA < 32M, then MVA = VA | (PID << 25), 
  otherwise VA = MVA. 
  32M = 2^25, so PID needs to be shifted left by 25 bits to get a non-duplicate physical address space.

Tip: VA < 32M, as for why it is 32, I think it may be because the code space of the program will not exceed the virtual address space of the following 32M when it is running. Since this method of obtaining MVA is to simplify the work of preventing different processes with overlapping virtual addresses from being mapped to the same physical address space, and the virtual memory space after 32M stores system components and dynamic link libraries, etc., which can be shared by different processes, there is no need to distinguish the physical address space corresponding to the virtual address space after 32M for each process.

• 2. Page table classification

• 2.1. Level 1 page table entries 
  

• Identifies the lower 2 bits	Entry Type
  00	Invalid entry
  01	Coarse page table entries
  10	Segment Entries
  11	Fine page table entries

• 2.2. Secondary page table entries 
  

• Identifies the lower 2 bits	Entry Type
  00	Invalid entry
  01	Huge Page Entries
  10	Minipage Entries
  11	Minipage entries

• 3. Relationship between page tables

• 

• 3.1、Directional relationship 
  

• It can be understood that segments, large pages, small pages, and tiny pages belong to the same level, because they can directly obtain physical addresses. 
  Coarse page tables and fine page tables belong to the second-level page table, and page tables belong to the first-level page table.

• 3.2. Inclusion Relationship 
  (1) A page table can contain 4096 segment entries (i.e., segment descriptors), and each segment stores 1M of physical address space.

• (2) The page table can contain 4096 coarse page tables. 
  1. Each coarse page table contains 256 entries, and each entry can index a 4K physical address. 
  2. A large page can index a 64K physical address, and every 16 entries in the coarse page table can point to the same large page 
  . 3. A small page can index a 4K physical address, and each entry in the coarse page table can point to a small page. 
  4. A tiny page can index a 1K physical address, and the coarse page table cannot point to a tiny page.

• (3) The page table can contain 4096 fine page tables. 
  1. Each fine page table contains 1024 entries, and each entry can index a 1K physical address. 
  2. A large page can index a 64K physical address, and every 64 entries in the fine page table can point to the same large page 
  . 3. A small page can index a 4K physical address, and every 4 entries in the fine page table can point to a small page. 
  4. A tiny page can index a 1K physical address, and every 1 entry in the fine page table can point to a tiny page.

• 4. Indexing steps

• (1) The segment mode 
  obtains the physical address of the first-level page table (bits [31:14]) from C2 (page table base address register) of CP15, and uses bits [31:20] of MVA to index the corresponding segment descriptor in the first-level page table. The physical address of the corresponding segment is obtained from bits [31:20] of the segment descriptor, and bits [19:0] of MVA index the physical address to be operated in the segment.

• (2) Coarse page table mode (large pages are stored): 
  Get the physical address of the first-level page table (bits [31:14]) from C2 (page table base address register) of CP15, and use bits [31:20] of MVA to index the corresponding coarse page table descriptor in the first-level page table. Use bits [31:10] of the coarse page table descriptor to get the physical address of the corresponding coarse page table. Use bits [19:12] of MVA to index a large page descriptor in the coarse page table. Use bits [31:16] of the large page descriptor to get the physical address of the large page. Use bits [15:0] of MVA to index the physical address to be operated in the large page. 
  The indexing method of large pages stored in fine page tables is similar.

• (3) Coarse page table mode (small pages are stored) 
  obtains the physical address of the first-level page table (bits [31:14]) from C2 (page table base address register) of CP15, and uses bits [31:20] of MVA to index the corresponding coarse page table descriptor in the first-level page table. Use bits [31:10] of the coarse page table descriptor to obtain the physical address of the corresponding coarse page table. Use bits [19:12] of MVA to index a small page descriptor in the coarse page table, take out descriptor [31:12] to obtain the physical address of the small page table, and then use bits [11:0] of MVA to index the physical address stored in the small page table.

• (4) The fine page table (which stores the tiny page table) 
  obtains the physical address of the first-level page table (bits [31:14]) from C2 (page table base address register) of CP15, and uses bits [31:20] of MVA to index the corresponding fine page table descriptor in the first-level page table. Bits [31:12] of the fine page table descriptor index into the base address of a tiny page table, and then bits [19:10] of MVA index into a descriptor in the tiny page table. [31:10] of the descriptor is taken out to index into the physical address of the tiny page, and then bits [9:0] of MVA index into the physical address to be operated in the tiny page.

• Summary:

• Segment index: 
  TTB[31:14] MVA[31:20] Get segment descriptor 
  DUB[31:20] MVA[19:0] Get physical address

• Coarse page table large page: 
  TTB[31:14] MVA[31:20] Get the coarse page table descriptor 
  SOB[31:10] MVA[19:12] Get the large page table descriptor 
  BIB[31:16] MVA[15:0] Get the physical address

• Fine page table large page: 
  TTB[31:14] MVA[31:20] Get fine page table descriptor 
  FIB[31:12] MVA[19:12] Get large page table descriptor 
  BIB[31:16] MVA[15:0] Get physical address

• Coarse page table small page: 
  TTB[31:14] MVA[31:20] Get the coarse page table descriptor 
  SOB[31:10] MVA[19:12] Get the small page table descriptor 
  SMB[31:12] MVA[11:0] Get the physical address

• Fine page table small page: 
  TTB[31:14] MVA[31:20] Get fine page table descriptor 
  FIB[31:12] MVA[19:12] Get small page table descriptor 
  SMB[31:12] MVA[11:0] Get physical address

• Fine page table tiny page: 
  TTB[31:14] MVA[31:20] Get fine page table descriptor 
  FIB[31:12] MVA[19:10] Get tiny page descriptor 
  VSB[31:10] MVA[9:0] Get physical address

• Tip:

• SOB: coarse base 
  FIB: fine base 
  DUB: section base 
  SMB: small base 
  VSB: very small base 
  BIB: big base