Memory Layout

x86-64 guest physical memory map

This is a survey of the existing memory layout for crosvm on x86-64 when booting a Linux kernel. Some of these values are different when booting a BIOS image; see the source. All addresses are in hexadecimal.

Name/source link	Address	End (exclusive)	Size	Notes
`START_OF_RAM_32BITS`	`0000`			RAM
`ZERO_PAGE_OFFSET`	`7000`			Linux boot_params structure
`BOOT_STACK_POINTER`	`8000`			Boot SP value
`boot_pml4_addr`	`9000`	`A000`	4 KiB	Boot page table
`boot_pdpte_addr`	`A000`	`B000`	4 KiB	Boot page table
`boot_pde_addr`	`B000`	`F000`	16 KiB	Boot page tables
`CMDLINE_OFFSET`	`2_0000`	`2_0800`	2 KiB	Linux kernel command line
`SETUP_DATA_START`	`2_0800`	`E_0000`	766 KiB	Linux kernel `setup_data` linked list
`ACPI_HI_RSDP_WINDOW_BASE`	`E_0000`			ACPI tables
`KERNEL_START_OFFSET`	`20_0000`			Linux kernel image load address
`initrd_start`	after kernel			Initial RAM disk for Linux kernel (optional)
`END_ADDR_BEFORE_32BITS`	after initrd	`D000_0000`	~3.24 GiB	RAM (<4G)
`PROTECTED_VM_FW_START`	`CFC0_0000`	`D000_0000`	4 MiB	pVM firmware (if running a protected VM)
`END_ADDR_BEFORE_32BITS`	`D000_0000`	`F400_0000`	576 MiB	Low (<4G) MMIO allocation area
`PCIE_CFG_MMIO_START`	`F400_0000`	`F800_0000`	64 MiB	PCIe enhanced config (ECAM)
`RESERVED_MEM_SIZE`	`F800_0000`	`1_0000_0000`	128 MiB	LAPIC/IOAPIC/HPET/…
`IDENTITY_MAP_ADDR`	`FEFF_C000`			Identity map segment
`TSS_ADDR`	`FEFF_D000`			Boot task state segment
	`1_0000_0000`			RAM (>4G)
	(end of RAM)			High (>4G) MMIO allocation area

aarch64 guest physical memory map

All addresses are IPA in hexadecimal.

Common layout

These apply for all boot modes.

Name/source link	Address	End (exclusive)	Size	Notes
`SERIAL_ADDR[3]`	`2e8`	`2f0`	8 bytes	Serial port MMIO
`SERIAL_ADDR[1]`	`2f8`	`300`	8 bytes	Serial port MMIO
`SERIAL_ADDR[2]`	`3e8`	`3f0`	8 bytes	Serial port MMIO
`SERIAL_ADDR[0]`	`3f8`	`400`	8 bytes	Serial port MMIO
`AARCH64_RTC_ADDR`	`2000`	`3000`	4 KiB	Real-time clock
`AARCH64_VMWDT_ADDR`	`3000`	`4000`	4 KiB	Watchdog device
`AARCH64_PCI_CAM_BASE_DEFAULT`	`1_0000`	`101_0000`	16 MiB	PCI configuration (CAM)
`AARCH64_VIRTFREQ_BASE`	`104_0000`	`105_0000`	64 KiB	Virtual cpufreq device
`AARCH64_PVTIME_IPA_START`	`1ff_0000`	`200_0000`	64 KiB	Paravirtualized time
`AARCH64_PCI_MEM_BASE_DEFAULT`	`200_0000`	`400_0000`	32 MiB	Low MMIO allocation area
`AARCH64_GIC_CPUI_BASE`	`3ffd_0000`	`3fff_0000`	128 KiB	vGIC
`AARCH64_GIC_DIST_BASE`	`3fff_0000`	`4000_0000`	64 KiB	vGIC
`AARCH64_PROTECTED_VM_FW_START`	`7fc0_0000`	`8000_0000`	4 MiB	pVM firmware (if running a protected VM)
`AARCH64_PHYS_MEM_START`	`8000_0000`		--mem size	RAM (starts at IPA = 2 GiB)
`plat_mmio_base`	after RAM	+0x800000	8 MiB	Platform device MMIO region
`high_mmio_base`	after plat_mmio	max phys addr		High MMIO allocation area

RAM Layout

The RAM layout depends on the --fdt-position setting, which defaults to start when load using --bios and to end when using --kernel.

In --kernel mode, the initrd is always loaded immediately after the kernel, with a 16 MiB alignment.

--fdt-position=start

Name/source link	Address	End (exclusive)	Size	Notes
`fdt_address`	`8000_0000`	`8020_0000`	2 MiB	Flattened device tree in RAM
`payload_address`	`8020_0000`			Kernel/BIOS load location in RAM

--fdt-position=after-payload

Name/source link	Address	End (exclusive)	Size	Notes
`payload_address`	`8000_0000`			Kernel/BIOS load location in RAM
`fdt_address`	after payload (2 MiB alignment)		2 MiB	Flattened device tree in RAM

--fdt-position=end

Name/source link	Address	End (exclusive)	Size	Notes
`payload_address`	`8000_0000`			Kernel/BIOS load location in RAM
`fdt_address`	before end of RAM (2 MiB alignment)		2 MiB	Flattened device tree in RAM

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