xc_domain_populate_physmap()

Overview

xenguest uses xc_domain_populate_physmap() to populate a Xen domain’s physical memory map: Both call the XENMEM_populate_physmap hypercall.

xc_domain_populate_physmap_exact() also sets the “exact” flag for allocating memory only on the given NUMA node. This is a very simplified overview of the hypercall:

flowchart LR

subgraph hypercall handlers
    populate_physmap("<tt>populate_physmap()</tt>
    One call for each memory
                      range (extent)")
end


subgraph "Xen buddy allocator:"

    populate_physmap
        --> alloc_domheap_pages("<tt>alloc_domheap_pages()</tt>
            Assign allocated pages to
            the domain")

    alloc_domheap_pages
        --> alloc_heap_pages("<tt>alloc_heap_pages()</tt>
                        If needed: split high-order
                        pages into smaller buddies,
                        and scrub dirty pages")
            --> get_free_buddy("<tt>get_free_buddy()</tt>
            If reqested: Allocate from a
            preferred/exact NUMA node
            and/or from
            unscrubbed memory
            ")

end

click populate_physmap
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/memory.c#L159-L314
" _blank

click alloc_domheap_pages
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L2641-L2697
" _blank

click get_free_buddy
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L855-L958
" _blank

click alloc_heap_pages
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L967-L1116
" _blank

memory_op(XENMEM_populate_physmap)

It calls construct_memop_from_reservation() to convert the arguments for allocating a page from struct xen_memory_reservation to struct memop_args and passes it to populate_physmap():

construct_memop_from_reservation()

It populates struct memop_args using the hypercall arguments. It:

• Copies extent_start, nr_extents, and extent_order.
• Populates memop_args->memflags using the given mem_flags.

Converting a vNODE to a pNODE for vNUMA

When a vNUMA vnode is passed using XENMEMF_vnode, and domain->vnuma and domain->vnuma->nr_vnodes are set, and the vnode (virtual NUMA node) maps to a pnode (physical NUMA node), it also:

• Populates the pnode in the memflags of the struct memop_args
• and sets a XENMEMF_exact_node_request in them as well.

propagate_node()

If no vNUMA node is passed, construct_memop_from_reservation() calls propagate_node() to propagate the NUMA node and XENMEMF_exact_node_request for use in Xen.

populate_physmap()

It handles hypercall preemption and resumption after a preemption, keeps track of the already populated pages.

For each range (extent), it runs an iteration of the allocation loop. It passes the

• struct domain
• page order
• count remaining pages populate
• and the converted memflags

to alloc_domheap_pages():

alloc_domheap_pages()

It calls alloc_heap_pages() and on success, assigns the allocated pages to the domain.

alloc_heap_pages()

It calls get_free_buddy()[^1] to allocate a page at the most suitable place: When no pages of the requested size are free, it splits larger superpages into pages of the requested size.

get_free_buddy()

It finds memory based on the flags and domain and return its page struct:

• Optionally allocate prefer to allocate from a passed NUMA node
• Optionally allocate from the domain’s next affine NUMA node (round-robin)
• Optionally return if the preferred NUMA allocation did not succeed
• Optionally allocate from not-yet scrubbed memory
• Optionally allocate from the given range of memory zones
• Fall back allocate from the next NUMA node on the system (round-robin)

For details see the get_free_buddy() finds memory based on the flags and domain.

Full flowchart

flowchart TD

subgraph XenCtrl
xc_domain_populate_physmap["<tt>xc_domain_populate_physmap()"]
xc_domain_populate_physmap_exact["<tt>xc_domain_populate_physmap_exact()"]
end

subgraph Xen

%% sub-subgraph from memory_op() to populate_node() and back

xc_domain_populate_physmap & xc_domain_populate_physmap_exact
<--reservation,<br>and for preempt:<br>nr_start/nr_done-->
memory_op("<tt>memory_op(XENMEM_populate_physmap)")

memory_op
    --struct xen_memory_reservation-->
        construct_memop_from_reservation("<tt>construct_memop_from_reservation()")
            --struct<br>xen_memory_reservation->mem_flags-->
                propagate_node("<tt>propagate_node()")
            --_struct<br>memop_args->memflags_-->
        construct_memop_from_reservation
    --_struct memop_args_-->
memory_op<--struct memop_args *:
            struct domain *,
            List of extent base addrs,
            Number of extents,
            Size of each extent (extent_order),
            Allocation flags(memflags)-->
    populate_physmap[["<tt>populate_physmap()"]]
        <-.domain, extent base addrs, extent size, memflags, nr_start and nr_done.->
        populate_physmap_loop--if memflags & MEMF_populate_on_demand -->guest_physmap_mark_populate_on_demand("
            <tt>guest_physmap_mark_populate_on_demand()")
        populate_physmap_loop@{ label: "While extents to populate,
                and not asked to preempt,
                for each extent left to do:", shape: notch-pent }
            --domain, order, memflags-->
            alloc_domheap_pages("<tt>alloc_domheap_pages()")
              --zone_lo, zone_hi, order, memflags, domain-->
                alloc_heap_pages
                    --zone_lo, zone_hi, order, memflags, domain-->
                        get_free_buddy("<tt>get_free_buddy()")
                    --_page_info_
                -->alloc_heap_pages
                    --if no page-->
                        no_scrub("<tt>get_free_buddy(MEMF_no_scrub)</tt>
                            (honored only when order==0)")
                    --_dirty 4k page_
                -->alloc_heap_pages
                    <--_dirty 4k page_-->
                        scrub_one_page("<tt>scrub_one_page()")
                alloc_heap_pages("<tt>alloc_heap_pages()</tt>
                        (also splits higher-order pages
                         into smaller buddies if needed)")
              --_page_info_
            -->alloc_domheap_pages
                --page_info, order, domain, memflags-->assign_page("<tt>assign_page()")
                    assign_page
                        --page_info, nr_mfns, domain, memflags-->
                            assign_pages("<tt>assign_pages()")
                            --domain, nr_mfns-->
                                domain_adjust_tot_pages("<tt>domain_adjust_tot_pages()")
            alloc_domheap_pages
            --_page_info_-->
        populate_physmap_loop
            --page(gpfn, mfn, extent_order)-->
                guest_physmap_add_page("<tt>guest_physmap_add_page()")

populate_physmap--nr_done, preempted-->memory_op
end

click memory_op
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/memory.c#L1409-L1425
" _blank

click construct_memop_from_reservation
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/memory.c#L1022-L1071
" _blank

click propagate_node
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/memory.c#L524-L547
" _blank

click populate_physmap
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/memory.c#L159-L314
" _blank

click populate_physmap_loop
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/memory.c#L197-L304
" _blank

click guest_physmap_mark_populate_on_demand
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L210-220
" _blank

click guest_physmap_add_page
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L296
" _blank

click alloc_domheap_pages
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L2641-L2697
" _blank

click alloc_heap_pages
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L967-L1116
" _blank

click get_free_buddy
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L855-L958
" _blank

click assign_page
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L2540-L2633
" _blank

click assign_pages
"https://github.com/xen-project/xen/blob/e16acd80/xen/common/page_alloc.c#L2635-L2639
" _blank