Here are some frequently asked questions about the ACRN project.

What hardware does ACRN support?

ACRN runs on Intel Apollo Lake and Kaby Lake boards, as documented in our Supported Hardware documentation.

Clear Linux* OS fails to boot on my NUC

If you’re following the Getting Started Guides documentation and the NUC fails to boot, here are some options to try:

  • Upgrade your platform to the latest BIOS
  • Verify Secure Boot is disabled in the BIOS settings:
    • Depending on your platform, press for example, F2 while booting to enter the BIOS options menu, and verify “Secure Boot” is not checked in the “Boot Options”
  • Make sure you are using EFI (and not legacy BIOS)

How do I configure ACRN’s memory use?

It’s important that the ACRN Kconfig settings are aligned with the physical memory on your platform. Check the documentation for these option settings for details:

For example, if memory is 32G, setup PLATFORM_RAM_SIZE = 32G

      hex "Size of the physical platform RAM"
      default 0x200000000 if PLATFORM_SBL
      default 0x800000000 if PLATFORM_UEFI

Setup SOS_RAM_SIZE = 32G too (The SOS will have the whole resource)

      hex "Size of the Service OS (SOS) RAM"
      default 0x200000000 if PLATFORM_SBL
      default 0x800000000 if PLATFORM_UEFI

Setup UOS_RAM_SIZE to what you need, for example, 16G

      hex "Size of the User OS (UOS) RAM"
      default 0x100000000 if PLATFORM_SBL
      default 0x400000000 if PLATFORM_UEFI

Setup HV_RAM_SIZE (we will reserve memory for guest EPT paging table), if you setup 32G (default 16G), you must enlarge it with (32G-16G)/2M pages (where pages are 4K). The example below is after HV_RAM_SIZE is changed to 240M

config HV_RAM_SIZE
  hex "Size of the RAM region used by the hypervisor"
  default 0x07800000 if PLATFORM_SBL
  default 0x0f000000 if PLATFORM_UEFI

How to modify the default display output for a UOS?

Apollo Lake HW has three pipes and each pipe can have three or four planes which help to display the overlay video. The hardware can support up to 3 monitors simultaneously. Some parameters are available to control how display monitors are assigned between the SOS and UOS(s), simplifying the assignment policy and providing configuration flexibility for the pipes and planes for various IoT scenarios. This is known as the plane restriction feature.

  • i915.avail_planes_per_pipe: for controlling how planes are assigned to the pipes
  • i915.domain_plane_owners: for controlling which domain (VM) will have access to which plane

Refer to GVT-g (AcrnGT) Kernel Options details for detailed parameter descriptions.

In the default configuration, pipe A is assigned to the SOS and pipes B and C are assigned to the UOS, as described by these parameters:

  • SOS:

  • UOS:


To assign pipes A and B to the UOS, while pipe C is assigned to the SOS, use these parameters:

  • SOS:

  • UOS:



The careful reader may have noticed that in all examples given above, the SOS always has at least one plane per pipe. This is intentional, and the driver will enforce this if the parameters do not do this.

Why does ACRN need to know how much RAM the system has?

Configuring ACRN at compile time with the system RAM size is a tradeoff between flexibility and functional safety certification. For server virtualization, one binary is typically used for all platforms with flexible configuration options given at run time. But, for IoT applications, the image is typically configured and built for a particular product platform and optimized for that product.

Important features for ACRN include Functional Safety (FuSa) and real-time behavior. FuSa requires a static allocation policy to avoid the potential of dynamic allocation failures. Real-time applications similarly benefit from static memory allocation. This is why ACRN removed all malloc()-type code, and why it needs to pre-identify the size of all buffers and structures used in the Virtual Memory Manager. For this reason, knowing the available RAM size at compile time is necessary to statically allocate memory usage.

How to build ACRN on Fedora 29?

There is a known issue when attempting to build ACRN on Fedora 29 because of how gnu-efi is packaged in this Fedora release. (See the ACRN GitHub issue for more information.) The following patch to /efi-stub/Makefile fixes the problem on Fedora 29 development systems (but should not be used on other Linux distros):

diff --git a/efi-stub/Makefile b/efi-stub/Makefile
index 5b87d49b..dfc64843 100644
--- a/efi-stub/Makefile
+++ b/efi-stub/Makefile
@@ -52,14 +52,14 @@ endif
 # its tools and libraries in different folders. The next couple of
 # variables will determine and set the right path for both the
 # tools $(GNUEFI_DIR) and libraries $(LIBDIR)
-GNUEFI_DIR := $(shell find $(SYSROOT)/usr/lib* -name elf_$(ARCH)_efi.lds -type f | xargs dirname)
+GNUEFI_DIR := $(shell find $(SYSROOT)/usr/lib* -name elf_x64_efi.lds -type f | xargs dirname)
LIBDIR := $(subst gnuefi,,$(GNUEFI_DIR))
-CRT0 := $(GNUEFI_DIR)/crt0-efi-$(ARCH).o
-LDSCRIPT := $(GNUEFI_DIR)/elf_$(ARCH)_efi.lds
+CRT0 := $(GNUEFI_DIR)/crt0-efi-x64.o
+LDSCRIPT := $(GNUEFI_DIR)/elf_x64_efi.lds

 INCDIR := $(SYSROOT)/usr/include

-CFLAGS=-I. -I.. -I../hypervisor/include/arch/x86/guest -I$(INCDIR)/efi -I$(INCDIR)/efi/$(ARCH) \
+CFLAGS=-I. -I.. -I../hypervisor/include/arch/x86/guest -I$(INCDIR)/efi -I$(INCDIR)/efi/x64 \
                 -I../hypervisor/include/public -I../hypervisor/include/lib -I../hypervisor/bsp/include/uefi \
                 -DEFI_FUNCTION_WRAPPER -fPIC -fshort-wchar -ffreestanding \
                 -Wall -I../fs/ -D$(ARCH) -O2 \