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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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