211 lines
8.1 KiB
HTML
211 lines
8.1 KiB
HTML
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<title>Lecture 5/title>
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<html>
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<head>
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</head>
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<body>
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<h2>Address translation and sharing using page tables</h2>
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<p> Reading: <a href="../readings/i386/toc.htm">80386</a> chapters 5 and 6<br>
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<p> Handout: <b> x86 address translation diagram</b> -
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<a href="x86_translation.ps">PS</a> -
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<a href="x86_translation.eps">EPS</a> -
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<a href="x86_translation.fig">xfig</a>
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<br>
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<p>Why do we care about x86 address translation?
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<ul>
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<li>It can simplify s/w structure by placing data at fixed known addresses.
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<li>It can implement tricks like demand paging and copy-on-write.
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<li>It can isolate programs to contain bugs.
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<li>It can isolate programs to increase security.
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<li>JOS uses paging a lot, and segments more than you might think.
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</ul>
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<p>Why aren't protected-mode segments enough?
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<ul>
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<li>Why did the 386 add translation using page tables as well?
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<li>Isn't it enough to give each process its own segments?
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</ul>
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<p>Translation using page tables on x86:
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<ul>
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<li>paging hardware maps linear address (la) to physical address (pa)
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<li>(we will often interchange "linear" and "virtual")
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<li>page size is 4096 bytes, so there are 1,048,576 pages in 2^32
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<li>why not just have a big array with each page #'s translation?
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<ul>
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<li>table[20-bit linear page #] => 20-bit phys page #
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</ul>
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<li>386 uses 2-level mapping structure
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<li>one page directory page, with 1024 page directory entries (PDEs)
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<li>up to 1024 page table pages, each with 1024 page table entries (PTEs)
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<li>so la has 10 bits of directory index, 10 bits table index, 12 bits offset
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<li>What's in a PDE or PTE?
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<ul>
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<li>20-bit phys page number, present, read/write, user/supervisor
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</ul>
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<li>cr3 register holds physical address of current page directory
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<li>puzzle: what do PDE read/write and user/supervisor flags mean?
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<li>puzzle: can supervisor read/write user pages?
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<li>Here's how the MMU translates an la to a pa:
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<pre>
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uint
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translate (uint la, bool user, bool write)
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{
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uint pde;
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pde = read_mem (%CR3 + 4*(la >> 22));
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access (pde, user, read);
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pte = read_mem ( (pde & 0xfffff000) + 4*((la >> 12) & 0x3ff));
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access (pte, user, read);
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return (pte & 0xfffff000) + (la & 0xfff);
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}
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// check protection. pxe is a pte or pde.
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// user is true if CPL==3
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void
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access (uint pxe, bool user, bool write)
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{
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if (!(pxe & PG_P)
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=> page fault -- page not present
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if (!(pxe & PG_U) && user)
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=> page fault -- not access for user
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if (write && !(pxe & PG_W))
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if (user)
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=> page fault -- not writable
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else if (!(pxe & PG_U))
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=> page fault -- not writable
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else if (%CR0 & CR0_WP)
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=> page fault -- not writable
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}
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</pre>
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<li>CPU's TLB caches vpn => ppn mappings
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<li>if you change a PDE or PTE, you must flush the TLB!
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<ul>
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<li>by re-loading cr3
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</ul>
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<li>turn on paging by setting CR0_PE bit of %cr0
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</ul>
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Can we use paging to limit what memory an app can read/write?
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<ul>
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<li>user can't modify cr3 (requires privilege)
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<li>is that enough?
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<li>could user modify page tables? after all, they are in memory.
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</ul>
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<p>How we will use paging (and segments) in JOS:
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<ul>
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<li>use segments only to switch privilege level into/out of kernel
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<li>use paging to structure process address space
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<li>use paging to limit process memory access to its own address space
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<li>below is the JOS virtual memory map
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<li>why map both kernel and current process? why not 4GB for each?
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<li>why is the kernel at the top?
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<li>why map all of phys mem at the top? i.e. why multiple mappings?
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<li>why map page table a second time at VPT?
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<li>why map page table a third time at UVPT?
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<li>how do we switch mappings for a different process?
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</ul>
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<pre>
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4 Gig --------> +------------------------------+
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| | RW/--
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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: . :
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: . :
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: . :
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|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~| RW/--
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| Remapped Physical Memory | RW/--
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| | RW/--
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KERNBASE -----> +------------------------------+ 0xf0000000
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| Cur. Page Table (Kern. RW) | RW/-- PTSIZE
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VPT,KSTACKTOP--> +------------------------------+ 0xefc00000 --+
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| Kernel Stack | RW/-- KSTKSIZE |
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| - - - - - - - - - - - - - - -| PTSIZE
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| Invalid Memory | --/-- |
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ULIM ------> +------------------------------+ 0xef800000 --+
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| Cur. Page Table (User R-) | R-/R- PTSIZE
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UVPT ----> +------------------------------+ 0xef400000
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| RO PAGES | R-/R- PTSIZE
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UPAGES ----> +------------------------------+ 0xef000000
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| RO ENVS | R-/R- PTSIZE
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UTOP,UENVS ------> +------------------------------+ 0xeec00000
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UXSTACKTOP -/ | User Exception Stack | RW/RW PGSIZE
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+------------------------------+ 0xeebff000
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| Empty Memory | --/-- PGSIZE
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USTACKTOP ---> +------------------------------+ 0xeebfe000
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| Normal User Stack | RW/RW PGSIZE
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+------------------------------+ 0xeebfd000
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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. .
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. .
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. .
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|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|
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| Program Data & Heap |
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UTEXT --------> +------------------------------+ 0x00800000
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PFTEMP -------> | Empty Memory | PTSIZE
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UTEMP --------> +------------------------------+ 0x00400000
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| Empty Memory | PTSIZE
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0 ------------> +------------------------------+
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</pre>
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<h3>The VPT </h3>
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<p>Remember how the X86 translates virtual addresses into physical ones:
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<p><img src=pagetables.png>
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<p>CR3 points at the page directory. The PDX part of the address
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indexes into the page directory to give you a page table. The
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PTX part indexes into the page table to give you a page, and then
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you add the low bits in.
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<p>But the processor has no concept of page directories, page tables,
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and pages being anything other than plain memory. So there's nothing
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that says a particular page in memory can't serve as two or three of
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these at once. The processor just follows pointers:
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pd = lcr3();
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pt = *(pd+4*PDX);
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page = *(pt+4*PTX);
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<p>Diagramatically, it starts at CR3, follows three arrows, and then stops.
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<p>If we put a pointer into the page directory that points back to itself at
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index Z, as in
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<p><img src=vpt.png>
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<p>then when we try to translate a virtual address with PDX and PTX
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equal to V, following three arrows leaves us at the page directory.
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So that virtual page translates to the page holding the page directory.
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In Jos, V is 0x3BD, so the virtual address of the VPD is
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(0x3BD<<22)|(0x3BD<<12).
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<p>Now, if we try to translate a virtual address with PDX = V but an
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arbitrary PTX != V, then following three arrows from CR3 ends
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one level up from usual (instead of two as in the last case),
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which is to say in the page tables. So the set of virtual pages
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with PDX=V form a 4MB region whose page contents, as far
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as the processor is concerned, are the page tables themselves.
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In Jos, V is 0x3BD so the virtual address of the VPT is (0x3BD<<22).
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<p>So because of the "no-op" arrow we've cleverly inserted into
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the page directory, we've mapped the pages being used as
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the page directory and page table (which are normally virtually
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invisible) into the virtual address space.
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</body>
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