Tweet Share Virtualization capabilities were added to the latest revision of the Armv7-A architectures (with processors like Cortex A7 and A15), and this was extended further with Armv8-A (64bit). Since then, Arm has been improving virtualization support with incremental versions of the Armv8 architecture. This talk will give an overview of the features added.

1. The evolu on of Virtualiza on
in the Arm Architecture
Julien Grall <julien.grall@arm.com>
Xen Developer Summit 2018
© 2018 Arm Limited

2. 2 © 2018 Arm Limited

3. Use cases
3 © 2018 Arm Limited

4. Type of hypervisors
Figure: From wikipedia
4 © 2018 Arm Limited

5. ARMv8-A Privilege Model
Support both AArch32 and AArch64 execu on modes
32-64bit inter-working limited to excep on boundaries
AArch64 always has a higher privilege than AArch32
AArch64 state is a superset of AArch32 state
5 © 2018 Arm Limited

6. ARM virtualiza on
Introduced with the latest version of ARMv7 architecture
New hypervisor execu on state
Non-Secure world, higher privilege than EL1
6 © 2018 Arm Limited

7. Virtualiza on in a nutshell
Second stage of memory transla on
Adds an extra level of indirec on between guests and physical memory
TLBs are tagged by Virtual Machine ID (VMID)
Ability to trap access of most system registers
The hypervisor decides what it wants to trap
Can handle IRQs, FIQs and asynchronous aborts
The guest doesn’t see physical interrupts firing, for example
Guests can call into EL2 mode (HVC instruc on)
Allows para-virtualizated services
Standard architecture peripherals are virtualiza on-aware
GIC and mer have specific features to help virtualiza on
7 © 2018 Arm Limited

8. EL2: Not EL1++ (ARMv8.0-A)
EL2 is not a superset of NS-EL1
Orthogonal mode to EL1
Allows mul plexing of NS-EL1 guests on the hardware
Own transla on regime
Separate Stage-1 transla on, no Stage-2 transla on
It would be difficult to run Linux in EL2
Requires too many changes to be prac cal
EL2 could be used as a ”world switch”
Between guests (baremetal hypervisor/Type I)
Between host and guest (hosted hypervisor/Type II)
This makes the host a form of specialized guest.
8 © 2018 Arm Limited

9. Hypervisor architecture - Type I
EL1
EL0
EL2
Guest Kernel
Hypervisor
Guest Kernel Guest Kernel
Guest
Userspace Userspace Userspace
Guest Guest
9 © 2018 Arm Limited

10. Hypervisor architecture - Type II
EL1
EL0
EL2
HYP
Host Kernel
Switching Code
Guest Kernel Guest Kernel
Host
Userspace Userspace Userspace
Guest Guest
10 © 2018 Arm Limited

11. EL2 enhancement (ARMv8.1-A)
The Virtualiza on Host Extension (VHE) expands the capability of EL2:
Designed to improve the support of the Type-2 hypervisors
Allows the host OS to be run at EL2
The host OS requires minimal changes to run at EL2
User-space s ll runs at EL0
Host has no so ware running at EL1
AArch64 specific
EL2 becomes a strict superset of EL1
11 © 2018 Arm Limited

12. Hosted hypervisor architecture on a pla orm without VHE
EL1
EL0
EL2
HYP
Host Kernel
Switching Code
Guest Kernel Guest Kernel
Host
Userspace Userspace Userspace
Guest Guest
12 © 2018 Arm Limited

13. Hosted hypervisor architecture on a pla orm with VHE
EL1
EL0
EL2
Guest Kernel Guest Kernel
Host
Userspace Userspace Userspace
Guest Guest
Host Kernel + HYP
13 © 2018 Arm Limited

14. Nested Virtualiza on (ARMv8.3-A)
The Nested Virtualiza on extension allows a hypervisor in a VM.
Unmodified guest hypervisor running in NS EL1
Implementa on of a host hypervisor required
Running at EL2
AArch64 specific
14 © 2018 Arm Limited

15. 15 © 2018 Arm Limited

16. Nested Virtualiza on on Arm
16 © 2018 Arm Limited

17. Nested Virtualiza on (ARMv8.3-A)
17 © 2018 Arm Limited

18. Nested Virtualiza on (Armv8.4.A)
ARMv8.4 extends Nested Virtualiza on to:
Reduce the number of traps
Improve performance of nested hypervisor
18 © 2018 Arm Limited

19. Memory Par oning And Monitoring (ARMv8.3-A)
The Memory Par oning And Monitoring (MPAM) allows:
to limit the memory system performance impact of a VM
provide more bandwidth to some processes
A hypervisor can monitor and control how VM uses:
memory of a system
communicate with other system components
19 © 2018 Arm Limited

20. Summary
Robust set of virtualiza on features
Not just about CPU virtualiza on
Covers the whole systems architecture
An architecture in mo on:
ARMv8.1-A: https://goo.gl/Ox4thV
ARMv8.2-A: https://goo.gl/0Ns37U
ARMv8.3-A: https://goo.gl/CJv1n0
ARMv8-4-A: https://goo.gl/tYZmhR
20 © 2018 Arm Limited

21. Ques ons?
21 © 2018 Arm Limited

22. The Arm trademarks featured in this presenta on are registered trademarks or
trademarks of Arm Limited (or its subsidiaries) in the US and/or elsewhere. All rights
reserved. All other marks featured may be trademarks of their respec ve owners.
www.arm.com/company/policies/trademarks
© 2018 Arm Limited