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回复 5# super皮波
/*
* Hardware-wise, we have a two level page table structure, where the first
* level has 4096 entries, and the second level has 256 entries. Each entry
* is one 32-bit word. Most of the bits in the second level entry are used
* by hardware, and there aren't any "accessed" and "dirty" bits.
*
* Linux on the other hand has a three level page table structure, which can
* be wrapped to fit a two level page table structure easily - using the PGD
* and PTE only. However, Linux also expects one "PTE" table per page, and
* at least a "dirty" bit.
*
* Therefore, we tweak the implementation slightly - we tell Linux that we
* have 2048 entries in the first level, each of which is 8 bytes (iow, two
* hardware pointers to the second level.) The second level contains two
* hardware PTE tables arranged contiguously, preceded by Linux versions
* which contain the state information Linux needs. We, therefore, end up
* with 512 entries in the "PTE" level.
*
* This leads to the page tables having the following layout:
*
* pgd pte
* | |
* +--------+
* | | +------------+ +0
* +- - - - + | Linux pt 0 |
* | | +------------+ +1024
* +--------+ +0 | Linux pt 1 |
* | |-----> +------------+ +2048
* +- - - - + +4 | h/w pt 0 |
* | |-----> +------------+ +3072
* +--------+ +8 | h/w pt 1 |
* | | +------------+ +4096
*
* See L_PTE_xxx below for definitions of bits in the "Linux pt", and
* PTE_xxx for definitions of bits appearing in the "h/w pt".
*
* PMD_xxx definitions refer to bits in the first level page table.
*
* The "dirty" bit is emulated by only granting hardware write permission
* iff the page is marked "writable" and "dirty" in the Linux PTE. This
* means that a write to a clean page will cause a permission fault, and
* the Linux MM layer will mark the page dirty via handle_pte_fault().
* For the hardware to notice the permission change, the TLB entry must
* be flushed, and ptep_set_access_flags() does that for us.
*
* The "accessed" or "young" bit is emulated by a similar method; we only
* allow accesses to the page if the "young" bit is set. Accesses to the
* page will cause a fault, and handle_pte_fault() will set the young bit
* for us as long as the page is marked present in the corresponding Linux
* PTE entry. Again, ptep_set_access_flags() will ensure that the TLB is
* up to date.
*
* However, when the "young" bit is cleared, we deny access to the page
* by clearing the hardware PTE. Currently Linux does not flush the TLB
* for us in this case, which means the TLB will retain the transation
* until either the TLB entry is evicted under pressure, or a context
* switch which changes the user space mapping occurs.
*/
这是pgtable-2levle.h中的注释 |
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发表于 2015-01-04 20:29
