Memory Setup

HVM boot memory setup

For HVM domains, hvm_build_setup_mem() is responsible for deriving the memory layout of the new domain, allocating the required memory and populating for the new domain. It must:

1. Derive the e820 memory layout of the system memory of the domain including memory holes depending on PCI passthrough and vGPU flags.
2. Load the BIOS/UEFI firmware images
3. Store the final MMIO hole parameters in the Xenstore
4. Call the libxenguest function xc_dom_boot_mem_init() (see below)
5. Call construct_cpuid_policy() to apply the CPUID featureset policy

It starts this by:

• Getting struct xc_dom_image, max_mem_mib, and max_start_mib.
• Calculating start and size of lower ranges of the domain’s memory maps
  • taking memory holes for I/O into account, e.g. mmio_size and mmio_start.
• Calculating lowmem_end and highmem_end.

Calling into libxenguest for the bootmem setup

hvm_build_setup_mem() then calls the libxenguest function xc_dom_boot_mem_init() to set up the boot memory of domains.

xl CLI also uses it to set up the boot memory of domains. It constructs the memory layout of the domain and allocates and populates the main system memory of the domain using calls to 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