下面是我在为godson CPU的页面可执行保护功能增加内核支持时分析linux-mips

mmu实现的一些笔记,没有时间整理,有兴趣就看看吧.也许第5节对整个工作过程的

分析会有些用,其他语焉不详的东西多数只是对我本人有点用.

   首先的,关键的,要明白MIPS CPU的tlb是软件管理的,cache也不是透明的,具体的

   参见他们的用户手册.

(for sgi-cvs kernel 2.4.17)

1. mmu context 

   cpu用8位asid来区分tlb表项所属的进程,但是进程超过256个怎么办?

   linux实现的思想是软件扩展,每256个一组,TLB任何时候只存放同一组的asid

   因此不会冲突. 从一组的某个进程转换到另一组时,把tlb刷新

   ASID switch

   include/asm/mmu_context.h:

     asid_cache: 

       8bit physical asid + software extension, the software extension bits

       are used as a version; this records the newest asid allocated,while

       process->mm->context records its own version.

     get_new_mmu_context: 

       asid_cache++, if increasement lead to change of software extension part

       then flush icache & tlb to avoid conflicting with old versions.

       asid_cache = 0 reserved to represent no valid mmu context case,so the 

       first asid_cache version start from 0x100.

     switch_mm:

       if asid version of new process differs from current process’,get a new

       context for it.( it’s safe even if it gets same 8bit asid as previous 

       because this process’ tlb entries must have been flushed at the time 

       of version increasement)

       set entryhi,install pgd

     activate_mm:

       get new asid,set to entryhi,install pgd.

2. pte bits

   页表的内容和TLB表项关系

   entrylo[01]: 

       3130 29                      6  5  3 2 1 0

       -------------------------------------------

       |   |     PFN                  | C  |D|V|G|

       -------------------------------------------

   r4k pte:

       31                12 111098 7 6 5  3 2 1 0

       -------------------------------------------

       |    PFN            | C  |D|V|G|B|M|A|W|R|P| 

       -------------------------------------------

    C: cache attr.

    D: Dirty

    V: valid

    G: global

    B: R4K_BUG

    M: Modified

    A: Accessed

    W: Write

    R: Read

    P: Present

    (last six bits implemented in software)

   godson entrylo:

       bit 30 is used as execution protect bit E,only bit25-6 are used

       as PFN.

       instruction fetch from a page has E cleared lead to address error

       exception.

   godson pte:

       31                12 111098 7 6 5  3 2 1 0

       -------------------------------------------

       |    PFN            | C  |D|V|G|E|M|A|W|R|P| 

       -------------------------------------------

   E: software implementation of execute protection.Page is executable when

      E is set,non-executable otherwise.(Notice,it is different from hardware

      bit 30 in entrylo)

3. actions dealing with pte

  pte_page: get page struct from its pte value

  pte_{none,present,read,write,dirty,young}: get pte status,use software bits

  pte_wrprotect: &= ~(_PAGE_WRITE | _PAGE_SILENT_WRITE)

  pte_rdprotect: &= ~(_PAGE_READ | _PAGE_SILENT_READ)

  pte_mkclean: &= ~(_PAGE_MODIFIED | _PAGE_SILENT_WRITE)

  pte_mkold: &= ~(_PAGE_ACCESSED | _PAGE_SILENT_READ)

  pte_mkwrite: |= _PAGE_WRITE && if (_PAGE_MODIFIED) |= _PAGE_SILENT_WRITE

  pte_mkread: |= _PAGE_READ && if (_PAGE_ACCESSED) |= _PAGE_SILENT_READ

  pte_mkdirty: |= _PAGE_MODIFIED && if (_PAGE_WRITE) |= _PAGE_SILENT_WRITE

  pte_mkyoung: |= _PAGE_ACCESSED && if (_PAGE_READ) |= _PAGE_SILENT_READ

  pgprot_noncached: (&~CACHE_MASK) | (_CACHE_UNCACHED) 

  mk_pte(page,prot): (unsigned long) ( page - memmap ) << PAGE_SHIFT | prot

  mk_pte_phys(physpage,prot): physpage | prot

  pte_modify(pte,prot): ( pte & _PAGE_CHG_MASK ) | newprot

  page_pte_{prot}: unused?

  set_pte: *ptep = pteval 

  pte_clear: set_pte(ptep,__pte(0));

  ptep_get_and_clear

  pte_alloc/free

4. exceptions

  tlb refill exception(0x80000000):

     (1) get badvaddr,pgd 

     (2) pte table ptr = badvaddr>>22 < 2 + pgd ,

     (3) get context,offset = context >> 1 & 0xff8 (bit 21-13 + three zero), 

     (4) load offset(pte table ptr) and offset+4(pte table ptr),

     *(5) right shift 6 bits,write to entrylo[01],

     (6) tlbwr

  tlb modified exception(handle_mod):

     (1) load pte,

     *(2) if _PAGE_WRITE set,set ACCESSED | MODIFIED | VALID | DIRTY,

          reload tlb,tlbwi

         else DO_FAULT(1)

  tlb load exception(handle_tlbl):

     (1) load pte

     (2) if _PAGE_PRESENT && _PAGE_READ, set ACCESSED | VALID

         else DO_FAULT(0)

  tlb store exception(handle_tlbs):

     (1) load pte

     *(2) if _PAGE_PRESENT && _PAGE_WRITE,set ACCESSED | MODIFIED | VALID | DIRTY

         else DO_FAULT(1)

   items marked with * need modification.

5. protection_map

   all _PXXX map to page_copy? Although vm_flags will at last make pte writeable

   as needed,but will this be inefficient? it seems that alpha is not doing so.

   mm setup/tear down:

     on fork,copy_mm:

       allocate_mm,

       memcpy(new,old)

                        slow path

       mm_init-->pgd_alloc-->pgd_init-->all point to invalid_pte

                          -->copy kseg pgds from init_mm

                       fast path: what’s the content of pgd?

                         --> point to invalid_pte too,see clear_page_tables

       dup_mmap->copy_page_range-->alloc page table entries and do cow if needed.

       copy_segmens--null

       init_new_context--set mm->context=0(allocate an array for SMP first) 

     on exec(elf file),load_elf_binary:

       flush_old_exec:

         exec_mmap

            exit_mmap(old_mm)

               free vm_area_struct

               zap_page_range: free pages

               clear_page_tables

                 pgd_clear: do nothing

                pmd_clear: set to invalid_pte

                pte_clear: set to zero

            mm_alloc

            initialize new mm( init_new_contex,add to list,activate it)

            mmput(oldmm)

      setup_arg_pages:

        initialize stack segment. mm_area_struct for stack segment is setup

        here.

      load elf image into the correct location in memory

         elf_prot generated from eppnt->p_flags 

         elf_map(..,elf_prot,..) 

           do_mmap

   a typical session for a user page to be read then written:

     (1) user allocates the space

     (2) kernel call do_mmap/do_brk, vm_area_struct created

     (3) user tries to read

     (4) tlb refill exception occurs,invalid_pte_table’s entry is loaded into 

         tlb

     (5) tlbl exception occurs, 

          do_page_fault(0)->handle_mm_fault(allocate pte_table)->handle_pte_fault

          -->do_no_page-->map to ZERO page,readonly,set_pte,update_mmu_cache

          (update_mmu_cache put new pte to tlb,NEED change for godson)

     (6) read done,user tries to write

     (7) tlbs exception occurs(suppose the tlb entry is not yet kicked out)

         because pte is write protected,do_page_fault(1) called.

         handle_mm_fault(find out the pte)-->handle_pte_fault->do_wp_page

         -->allocate page,copy page,break_cow-->make a writeable pte,

         -->establish_pte-->write pte and update_mmu_cache

     (8) write done.

   above has shown that handle_mm_fault doesn’t care much about what the 

   page_prot is. (Of course,it has to be reasonable)

   What really matters is vm_flags,it will decide whether an access is valid

6. do_page_fault

   seems ok

7. swapping

   seems ok

8. adding execution protection

 2002.3.16:

      TLB execute protection bit support.

        1. generic support

        idea: 

          use bit 5 in pte to maintain a software version of _PAGE_EXEC

          modify TLB refill code to reflect it into hardware bit(bit 30)

        affected files:

          include/asm/pgtable.h: 

            define _PAGE_EXEC

            change related PAGE_XXX macros and protection_map

            add pte_mkexec/pte_exprotect

            add godson_mkexec/godson_mkprotect

          arch/mips/mm/tlbex-r4k.S:

           tlb_refill exception & PTE_RELOAD macro:

            test bit 5 and translated it into bit30 in entrylo 

            using godson’s cp0 register 23/24 as temporary store place

            Note: bit5 and bit30 have adverse meaning,bit5 set==bit30 

                  cleared==page executable,

          arch/mips/mm/tlb-r4k.c:

            update_mmu_cache:

                test bit 5 and translated it into bit30 in entrylo 

            implement godson_mkexec/godson_exprotect

          arch/mips/config.in:

            add option CONFIG_CPU_HAS_EXECUTE_PROTECTION

        2. non-executable stack support

          interface:

           by default no protection is taken,To take advantage of 

           this support,one should call sysmips syscall to set the

           flag bit and then execute the target program.

          affected files:

           include/asm/processor.h:

                define MF_STACK_PROTECTION flag

           fs/exec.c:

                judge which protection to use

           arch/mips/kernel/signal.c:

                enable/disable execute for signal trampoline

           arch/mips/math-emu/cp1emu.c:

                enable/disable execute for delay slot emulation trampoline

           arch/mips/kernel/sysmips.c:

                handle MF_STACK_PROTECTION