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SubjectRe: [PATCH 05/10] xen/setup: Set identity mapping for non-RAM E820 and E820 gaps.
> Couldn't you just do something like:
>
> if (e820->map[i].type != E820_RAM)

I am going to assume you meant '==' here.

> 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?

A derivation of this does work:

last = ISA_END_ADDRESS;
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;

/* Skip over the 1MB region. */
if (last > end)
continue;

if (e820->map[i].type == E820_RAM) {
/* Without saving 'last' we would end up gobbling RAM regions. */
last = end;
continue;
}

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

last = end;
}


>
> 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?

The issue here is that when this code path is called (xen_memory_setup),
the p2m_top[topidx][mididx] has been set to start_info->mfn_list[] (by
xen_build_dynamic_phys_to_machine).

Granted, some of these entries have been evicted (by the xen_return_unused_memory),
so the regions in the mfn_list are pock-marked with INVALID_P2M_ENTRY. For those
regions we set the PFN in the p2m_top[topidx][mididx][idx]. We do
not allocate anything during this pass.

In the the dom0_mem=max:X (or X,max:Y, where Y>X), which I neglected to test this
would actually try to allocate (whoops). Let me roll up a patch for this.

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

Not sure if it is truly needed.


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