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    Subject[PATCH 03 of 12] xen: efficiently support a holey p2m table
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    When using sparsemem and memory hotplug, the kernel's pseudo-physical
    address space can be discontigious. Previously this was dealt with by
    having the upper parts of the radix tree stubbed off. Unfortunately,
    this is incompatible with save/restore, which requires a complete p2m
    table.

    The solution is to have a special distinguished all-invalid p2m leaf
    page, which we can point all the hole areas at. This allows the tools
    to see a complete p2m table, but it only costs a page for all memory
    holes.

    It also simplifies the code since it removes a few special cases.

    Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
    ---
    arch/x86/xen/mmu.c | 18 ++++++++++++------
    1 file changed, 12 insertions(+), 6 deletions(-)

    diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
    --- a/arch/x86/xen/mmu.c
    +++ b/arch/x86/xen/mmu.c
    @@ -57,8 +57,17 @@
    #include "mmu.h"

    #define P2M_ENTRIES_PER_PAGE (PAGE_SIZE / sizeof(unsigned long))
    +#define TOP_ENTRIES (MAX_DOMAIN_PAGES / P2M_ENTRIES_PER_PAGE)

    -static unsigned long *p2m_top[MAX_DOMAIN_PAGES / P2M_ENTRIES_PER_PAGE];
    +/* Placeholder for holes in the address space */
    +static unsigned long p2m_missing[P2M_ENTRIES_PER_PAGE]
    + __attribute__((section(".data.page_aligned"))) =
    + { [ 0 ... P2M_ENTRIES_PER_PAGE-1 ] = ~0UL };
    +
    + /* Array of pointers to pages containing p2m entries */
    +static unsigned long *p2m_top[TOP_ENTRIES]
    + __attribute__((section(".data.page_aligned"))) =
    + { [ 0 ... TOP_ENTRIES - 1] = &p2m_missing[0] };

    static inline unsigned p2m_top_index(unsigned long pfn)
    {
    @@ -92,9 +101,6 @@
    return INVALID_P2M_ENTRY;

    topidx = p2m_top_index(pfn);
    - if (p2m_top[topidx] == NULL)
    - return INVALID_P2M_ENTRY;
    -
    idx = p2m_index(pfn);
    return p2m_top[topidx][idx];
    }
    @@ -110,7 +116,7 @@
    for(i = 0; i < P2M_ENTRIES_PER_PAGE; i++)
    p[i] = INVALID_P2M_ENTRY;

    - if (cmpxchg(pp, NULL, p) != NULL)
    + if (cmpxchg(pp, p2m_missing, p) != p2m_missing)
    free_page((unsigned long)p);
    }

    @@ -129,7 +135,7 @@
    }

    topidx = p2m_top_index(pfn);
    - if (p2m_top[topidx] == NULL) {
    + if (p2m_top[topidx] == p2m_missing) {
    /* no need to allocate a page to store an invalid entry */
    if (mfn == INVALID_P2M_ENTRY)
    return;



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