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    SubjectRe: [PATCH 05/10] xen/setup: Set identity mapping for non-RAM E820 and E820 gaps.
    On 12/21/2010 01:37 PM, Konrad Rzeszutek Wilk wrote:
    > For all regions that are not considered RAM, we do not
    > necessarily have to set the P2M, as it assumes that any
    > P2M top branch (so covering 134217728 pages, on 64-bit) or
    > middle branch (so covering 262144 pages, on 64-bit) are identity.
    > Meaning pfn_to_mfn(pfn)==pfn. However, not all E820 gaps are
    > that large, so for smaller and for boundary conditions we fill
    > out the P2M mapping with the identity mapping.
    >
    > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
    > ---
    > arch/x86/xen/setup.c | 34 ++++++++++++++++++++++++++++++++++
    > 1 files changed, 34 insertions(+), 0 deletions(-)
    >
    > diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
    > index d984d36..752c865 100644
    > --- a/arch/x86/xen/setup.c
    > +++ b/arch/x86/xen/setup.c
    > @@ -146,6 +146,34 @@ static unsigned long __init xen_return_unused_memory(unsigned long max_pfn,
    > return released;
    > }
    >
    > +static unsigned long __init xen_set_identity(const struct e820map *e820)
    > +{
    > + phys_addr_t last = 0;
    > + int i;
    > + unsigned long identity = 0;
    > + unsigned long pfn;
    > +
    > + for (i = 0; i < e820->nr_map; i++) {
    > + phys_addr_t start = e820->map[i].addr;
    > + phys_addr_t end = start + e820->map[i].size;
    > +
    > + if (end < start)
    > + continue;
    > +
    > + if (e820->map[i].type != E820_RAM) {
    > + for (pfn = PFN_UP(start); pfn < PFN_DOWN(end); pfn++)
    > + set_phys_to_machine(pfn, pfn);
    > + identity += pfn - PFN_UP(start);
    > + }
    > + if (start > last && ((start - last) > 0)) {
    > + for (pfn = PFN_UP(last); pfn < PFN_DOWN(start); pfn++)
    > + set_phys_to_machine(pfn, pfn);
    > + identity += pfn - PFN_UP(last);
    > + }

    Couldn't you just do something like:

    if (e820->map[i].type != E820_RAM)
    continue;

    for (pfn = PFN_UP(last); pfn < PFN_DOWN(start); pfn++)
    set_phys_to_machine(pfn, pfn);
    identity += pfn - PFN_UP(last);

    last = end;

    ie, handle the hole and non-RAM cases together?

    Also, what happens with the p2m tree mid layers in this? If you're
    doing page-by-page set_phys_to_machine, won't it end up allocating them
    all? How can you optimise the "large chunks of address space are
    identity" case?

    It would probably be cleanest to have a set_ident_phys_to_machine(start,
    end) function which can do all that.

    J
    > + last = end;
    > + }
    > + return identity;
    > +}
    > /**
    > * machine_specific_memory_setup - Hook for machine specific memory setup.
    > **/
    > @@ -254,6 +282,12 @@ char * __init xen_memory_setup(void)
    >
    > xen_add_extra_mem(extra_pages);
    >
    > + /*
    > + * Set P2M for all non-RAM pages and E820 gaps to be identity
    > + * type PFNs.
    > + */
    > + printk(KERN_INFO "Set %ld page(s) to 1-1 mapping.\n",
    > + xen_set_identity(&e820));
    > return "Xen";
    > }
    >



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