[guest] Add nested virtualization instructions to the README
Change-Id: Ia1bca5e8745e6060878c6c3d776ddad017e57eae
diff --git a/garnet/bin/guest/README.md b/garnet/bin/guest/README.md
index 32cd80e..a988175 100644
--- a/garnet/bin/guest/README.md
+++ b/garnet/bin/guest/README.md
@@ -3,7 +3,7 @@
hypervisor. The hypervisor and VMM are collectively referred to as "Machina".
These instructions will guide you through creating minimal Zircon and Linux
-guests. For instructions on building a more comprehensive linux guest system
+guests. For instructions on building a more comprehensive Linux guest system
see the [debian_guest](pkg/debian_guest/README.md) package.
These instructions assume a general familiarity with how to netboot the target
@@ -19,6 +19,9 @@
Where `${ARCH}` is one of `x64` or `arm64`.
### Note for external developers
+(Googlers: You don't need to do this, the Linux images are downloaded from CIPD
+by jiri.)
+
The linux_guest package expects the Linux kernel binaries to be in
`garnet/bin/guest/pkg/linux_guest`, you should create them before running
`fx full-build` by running the following scripts:
@@ -56,18 +59,37 @@
$ fx run -k
```
-Running a guest on QEMU on arm64 requires either using GICv2 (pass `-G 2`).
+You may also need to enable nested KVM on your host machine. The following
+instructions assume a Linux host machine with an Intel processor.
+
+To check whether nested virtualization is enabled (Y = enabled, 0 or N = not
+enabled):
+```
+$ cat /sys/module/kvm_intel/parameters/nested
+```
+
+To enable nested virtualization until the next reboot:
+```
+$ modprobe -r kvm_intel
+$ modprobe kvm_intel nested=1
+```
+To make the change permanent add the following line to
+`/etc/modprobe.d/kvm.conf`
+```
+options kvm_intel nested=1
+```
+
+Running an arm64 guest on QEMU requires either using GICv2 (pass `-G 2`).
```
$ fx run -G 2
```
Or using a more recent version of QEMU (try 2.12.0). Older versions of QEMU do
-not correctly emulate GICv3 when running with multiple guest VCPUs. If you do
-this, then you will need to launch the guest with `gic=3`.
+not correctly emulate GICv3 when running with multiple guest VCPUs.
```
$ fx run -q /path/to/recent/qemu/aarch64-softmmu
...
-$ guest launch (linux_guest|zircon_guest) --gic=3
+$ guest launch (linux_guest|zircon_guest)
```
## Running from Topaz
