xc_dom_boot_mem_init()

VM boot memory setup

xenguest’s hvm_build_setup_mem() and libxl and the xl CLI call xc_dom_boot_mem_init() to allocate and populate the domain’s system memory for booting it:

flowchart LR

subgraph libxl / xl CLI
    libxl__build_dom("libxl__build_dom()")
end

subgraph xenguest
    hvm_build_setup_mem("hvm_build_setup_mem()")
end

subgraph libxenctrl
    xc_domain_populate_physmap("One call for each memory range (extent):
    xc_domain_populate_physmap()
    xc_domain_populate_physmap()
    xc_domain_populate_physmap()")
end

subgraph libxenguest

    hvm_build_setup_mem & libxl__build_dom
        --> xc_dom_boot_mem_init("xc_dom_boot_mem_init()")

    xc_dom_boot_mem_init
        --> meminit_hvm("meminit_hvm()") & meminit_pv("meminit_pv()")
            --> xc_domain_populate_physmap
end

click xc_dom_boot_mem_init
"https://github.com/xen-project/xen/blob/39c45c/tools/libs/guest/xg_dom_boot.c#L110-L126
" _blank

click meminit_hvm
"https://github.com/xen-project/xen/blob/39c45c/tools/libs/guest/xg_dom_x86.c#L1348-L1648
" _blank

click meminit_pv
"https://github.com/xen-project/xen/blob/de0254b9/tools/libs/guest/xg_dom_x86.c#L1183-L1333
" _blank

The allocation strategies of them called functions are:

Strategy of the libxenguest meminit functions

• Attempt to allocate 1GB superpages when possible
• Fall back to 2MB pages when 1GB allocation failed
• Fall back to 4k pages when both failed

They use xc_domain_populate_physmap() to perform memory allocation and to map the allocated memory to the system RAM ranges of the domain.

Strategy of xc_domain_populate_physmap()

xc_domain_populate_physmap() calls the XENMEM_populate_physmap command of the Xen memory hypercall.

For a more detailed walk-through of the inner workings of this hypercall, see the reference on xc_domain_populate_physmap().

For more details on the VM build step involving xenguest and Xen side see: https://wiki.xenproject.org/wiki/Walkthrough:_VM_build_using_xenguest