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    [S390] Virtual memmap for s390. · f4eb07c1
    Heiko Carstens 提交于
    Virtual memmap support for s390. Inspired by the ia64 implementation.
    
    Unlike ia64 we need a mechanism which allows us to dynamically attach
    shared memory regions.
    These memory regions are accessed via the dcss device driver. dcss
    implements the 'direct_access' operation, which requires struct pages
    for every single shared page.
    Therefore this implementation provides an interface to attach/detach
    shared memory:
    
    int add_shared_memory(unsigned long start, unsigned long size);
    int remove_shared_memory(unsigned long start, unsigned long size);
    
    The purpose of the add_shared_memory function is to add the given
    memory range to the 1:1 mapping and to make sure that the
    corresponding range in the vmemmap is backed with physical pages.
    It also initialises the new struct pages.
    
    remove_shared_memory in turn only invalidates the page table
    entries in the 1:1 mapping. The page tables and the memory used for
    struct pages in the vmemmap are currently not freed. They will be
    reused when the next segment will be attached.
    Given that the maximum size of a shared memory region is 2GB and
    in addition all regions must reside below 2GB this is not too much of
    a restriction, but there is room for improvement.
    Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
    Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>
    f4eb07c1
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