IA32 Paging SchemeA quick review‘Traditional’ LinuxHow ‘mappings’ workControl Register 4Two-Level Translation SchemeControl Register 3Your ‘/proc/cr3’ pseudo-fileFormat of a Page-Directory entryFormat of a Page-Table entrykernel mappingsNormal page-mappingMapping ‘high’ memoryA limited number of kmapsPKMAP_BASEIn-class exercise #1In-class exercise #2IA32 Paging SchemeIntroduction to the Pentium’s support for “virtual” memoryA quick review•We’ve learned how to create ‘/proc’ files •We wrote a simple device-driver: ‘dram.c’•We displayed a task’s page-directory•We looked at Linux’s ‘mem_map[]’ array•Now we want to understand the ‘kmap()’ function, used by modules to get a virtual address for any physical page-frame‘Traditional’ LinuxUserspaceVirtual addressesKernelspace NORMALMEMORYHIGH MEMORYPhysical addresses0x000000000x38000000(896MB) Most of the kernel’s 1GB address-space is directly mapped to physical RAM at all times1G3GBut mappings to the ‘high memory’ page-framesare, by definition, only temporarily available andare not mapped according to a predictable ruleHow ‘mappings’ work•There are two aspects to consider:–the Pentium’s address-translation mechanism–the policies Linux applies to CPU capabilities•Example of this distinction:–The ‘page-directory’ array is a Pentium object–The ‘mem_map’ array is a Linux data-objectControl Register 431 0PAEPSE45This register consists of bits that enable various architecturalextensions to the standard features of the Pentium processor,including two that affect virtual-to-physical memory mapping. Legend: PSE = Page-Size Extensions (1=enabled, 0=disabled) PAE = Page-Address Extensions (1=enabled, 0=disabled) Recall that we saw our classroom workstations had CR4 = 000006D0Two-Level Translation Scheme PAGEDIRECTORYCR3 PAGETABLES PAGEFRAMESControl Register 3PWTPCDPage-directory base-address31 12 4 3PWT = Page Write-Through (1=yes, 0 = no)PCD = Page Cache-Disable (1 = yes, 0 = no)The ‘page-directory base-address’ field holds the upper 20 address-bits of a physical page-frame that contains the current task’s page-directory entries;the lower 12 address-bits of any page-frame are zeros, of course, so can beused by the processor for other purposes (i.e., PCD [bit 4] and PWT [bit 3]:Your ‘/proc/cr3’ pseudo-file•Recall that you created a kernel module (named ‘cr3.c’) which allows applications to obtain the current value in register CR3•This CR3 value gives the physical address for the current task’s page-directory, if the ‘traditional’ 3G/1G user-kernel mapping is employed (but not with the 4G/4G “patch”)•Using ‘fileview’ you saw a page-directory!Format of a Page-Directory entryPAGE-TABLE BASE ADDRESS PWUPWTPCDA0PSG31 12 11 10 9 8 7 6 5 4 3 2 1 0AVAILLEGEND P = Present (1=yes, 0=no) W = Writable (1 = yes, 0 = no) U = User (1 = yes, 0 = no) A = Accessed (1 = yes, 0 = no) G = Global (1 = yes, 0 = no)PWT = Page Write-Through (1=yes, 0 = no)PCD = Page Cache-Disable (1 = yes, 0 = no)PS = Page-Size (0=4KB, 1 = 4MB)Format of a Page-Table entryPAGE-FRAME BASE ADDRESS PWUPWTPCDAD0G31 12 11 10 9 8 7 6 5 4 3 2 1 0AVAILLEGEND P = Present (1=yes, 0=no) W = Writable (1 = yes, 0 = no) U = User (1 = yes, 0 = no) A = Accessed (1 = yes, 0 = no) D = Dirty (1 = yes, 0 = no) G = Global (1 = yes, 0 = no)PWT = Page Write-Through (1=yes, 0 = no)PCD = Page Cache-Disable (1 = yes, 0 = no)kernel mappings•The ‘kmap()’ function is used by modules to obtain the virtual address for a physical page-frame:void * kmap( struct page * page );•On systems with 1GB of installed memory, the majority of physical page-frames are permanently mapped to kernel addresses, but page-frames in ‘high’ memory can only be ‘temporarily’ mapped into kernel spaceNormal page-mapping•For a virtual address in kernel space that is below the high-memory start-address, the corresponding physical address can be gotten by a simple subtraction:phys_addr = virt_addr – PAGE_OFFSET•Recall: PAGE_OFFSET = 0xC0000000Mapping ‘high’ memory•For addresses in kernel space in the rangefrom 0xF8000000 to 0xFFFFFFFFthere might be no current mapping to ram •So the action of the ‘kmap()’ function must be more complex in these cases (i.e., it’s necessary to create a temporary mapping if it happens one does not already exist)A limited number of kmaps•In kernel 2.6 there is only one page-table that is reserved for ‘kmap()’ to use when temporary mappings into high-memory need to be created•This is called the ‘kmap_page_table’ •Its page-table entries are forever changing as kernel code calls the functions ‘kmap()’ and ‘kunmap()’PKMAP_BASE•We can watch the ‘kmap_page_table’ as it undergoes these dynamic changes – if we know where to find it in physical memory•We can do a ‘page-table walk’ to locate it•There are 1024 virtual page-frames used for temporary ‘kmap()’ addresses, starting with the virtual address PKMAP_BASEIn-class exercise #1•Use the ‘init_mm.c’ module to locate the page-directory belonging to the ‘init’ task•Use ‘fileview’ to see these directory entries•Determine the value of PKMAP_BASE •Which directory-entry contains the address of the ‘pkmap_page_table’? (It’s the entry that ‘maps’ virtual-address PKMAP_BASE to a page-frame in physical memory)In-class exercise #2•Use the result from your ‘page-table walk’ to assign the correct value to ‘zone_base’ in our ‘kmapsnow.cpp’ demo-program•Run the ‘kmapsnow’ in one window while you execute some other commands from inside a different window, to view changes in the set of currently active kmaps•Log in remotely from an adjacent