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docs/arm64: Document the hardware constraints

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There is a corner case of using the Cloud Hypervisor on AArch64:
If the VM is started on a device where RAM is limited, and if the user
allocates nearly as much memory for the guest as is still free on the
host, we need to enable the swap memory.

This commit documented this corner case with explanation.

Also, this commit corrects the hardware requirement of the GIC interrupt
controller for running the Cloud Hypervisor on AArch64, accroding to
[1].

Fixes: https://github.com/cloud-hypervisor/cloud-hypervisor/issues/3419

Signed-off-by: Henry Wang <Henry.Wang@arm.com>

[1] https://www.kernel.org/doc/html/latest/virt/kvm/devices/arm-vgic-its.html
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Henry Wang 2021-12-24 00:26:09 -05:00 committed by Michael
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docs/arm64.md
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# How to build and test Cloud Hypervisor on AArch64
This document introduces how to build and test Cloud Hypervisor on AArch64
servers. Currently Cloud Hypervisor cannot be tested on Raspberry PI. Because
on AArch64, Cloud Hypervisor requires GICv3-ITS device for PCIe MSI interrupt
handling. But GICv3-ITS has not been equipped on any Raspberry PI product so
far.
Now Cloud Hypervisor supports 2 ways of booting on AArch64: UEFI booting and
direct-kernel booting. The document covers both of the ways.
This document introduces how to build and test Cloud Hypervisor on AArch64.
Currently, Cloud Hypervisor supports 2 methods of booting on AArch64: UEFI
booting and direct-kernel booting. The document covers both methods.
All the steps are based on Ubuntu. We use the Ubuntu cloud image for guest VM
disk.
## Hardware requirements
- AArch64 servers (recommended) or development boards equipped with the GICv3
interrupt controller.
- On development boards that have constrained RAM resources, if the creation of
a VM consumes a large portion of the free memory on the host, it may be required
to enable swap. For example, this was required on a board with 3 GB of RAM
booting a 2 GB VM at a point in time when 2.8 GB were free. Without enabling
swap the `cloud-hypervisor` process was terminated by the OOM killer. In this
situation memory was allocated for the virtual machine using memfd while the
page cache was filled, leading to a situation where the kernel could not even
drop caches. Making a small section of swap available (observably, 1 to 15 MB),
this situation can be resolved and the resulting memory footprint of
`cloud-hypervisor` is as expected.
## Getting started
We create a folder to build and run Cloud Hypervisor at `$HOME/cloud-hypervisor`