sysctsl: Add how-to doc for setting sysctls.

Document sysctls for Docker and Kubernetes.

Fixes #399

Signed-off-by: Archana Shinde <archana.m.shinde@intel.com>

how-to/how-to-use-sysctls-with-kata.md

@@ -0,0 +1,143 @@
+# Setting Sysctls with Kata
+
+## Sysctls
+In Linux, the sysctl interface allows an administrator to modify kernel 
+parameters at runtime. Parameters are available via the `/proc/sys/` virtual 
+process file system. 
+
+The parameters include the following subsystems among others:
+- `fs` (file systems)
+- `kernel` (kernel)
+- `net` (networking)
+- `vm` (virtual memory)
+
+To get a complete list of kernel parameters, run:
+```
+$ sudo sysctl -a
+```
+
+Both Docker and Kubernetes provide mechanisms for setting namespaced sysctls. 
+Namespaced sysctls can be set per pod in the case of Kubernetes or per container
+in case of Docker.
+The following sysctls are known to be namespaced and can be set with 
+Docker and Kubernetes:
+
+- `kernel.shm*`
+- `kernel.msg*`
+- `kernel.sem`
+- `fs.mqueue.*`
+- `net.*`
+
+### Namespaced Sysctls:
+
+Kata Containers supports setting namespaced sysctls with Docker and Kubernetes.
+All namespaced sysctls can be set in the same way as regular Linux based
+containers, the difference being, in the case of Kata they are set inside the guest.
+
+#### Setting Namespaced Sysctls with Docker:
+
+```
+$ sudo docker run --runtime=kata-runtime -it alpine cat /proc/sys/fs/mqueue/queues_max
+256
+$ sudo docker run --runtime=kata-runtime --sysctl fs.mqueue.queues_max=512 -it alpine cat /proc/sys/fs/mqueue/queues_max
+512
+```
+
+... and:
+
+```
+$ sudo docker run --runtime=kata-runtime -it alpine cat /proc/sys/kernel/shmmax
+18446744073692774399
+$ sudo docker run --runtime=kata-runtime --sysctl kernel.shmmax=1024 -it alpine cat /proc/sys/kernel/shmmax
+1024
+```
+
+For additional documentation on setting sysctls with Docker please refer to [Docker-sysctl-doc](https://docs.docker.com/engine/reference/commandline/run/#configure-namespaced-kernel-parameters-sysctls-at-runtime).
+
+
+#### Setting Namespaced Sysctls with Kubernetes:
+
+Kubernetes considers certain sysctls as safe and others as unsafe. For detailed
+information about what sysctls are considered unsafe, please refer to the [Kubernetes sysctl docs](https://kubernetes.io/docs/tasks/administer-cluster/sysctl-cluster/).
+For using unsafe systcls, the cluster admin would need to allow these as:
+
+```
+$ kubelet --allowed-unsafe-sysctls 'kernel.msg*,net.ipv4.route.min_pmtu' ...
+```
+
+or using the declarative approach as:
+
+```
+$ cat kubeadm.yaml
+apiVersion: kubeadm.k8s.io/v1alpha3
+kind: InitConfiguration
+nodeRegistration:
+  kubeletExtraArgs:
+    allowed-unsafe-sysctls: "kernel.msg*,kernel.shm.*,net.*"
+...
+```
+
+The above yaml can then be passed to `kubeadm init` as:
+```
+$ sudo -E kubeadm init --config=kubeadm.yaml
+```
+
+Both safe and unsafe sysctls can be enabled in the same way in the Pod yaml:
+
+```
+apiVersion: v1
+kind: Pod
+metadata:
+  name: sysctl-example
+spec:
+  securityContext:
+    sysctls:
+    - name: kernel.shm_rmid_forced
+      value: "0"
+    - name: net.ipv4.route.min_pmtu
+      value: "1024"
+```
+
+### Non-Namespaced Sysctls:
+
+Docker and Kubernetes disallow sysctls without a namespace.
+The recommendation is to set them directly on the host or use a privileged
+container in the case of Kubernetes.
+
+In the case of Kata, the approach of setting sysctls on the host does not
+work since the host sysctls have no effect on a Kata Container running 
+inside a guest. Kata gives you the ability to set non-namespaced sysctls using a privileged container.
+This has the advantage that the non-namespaced sysctls are set inside the guest 
+without having any effect on the `/proc/sys` values of any other pod or the 
+host itself. 
+
+The recommended approach to do this would be to set the sysctl value in a
+privileged init container. In this way, the application containers do not need any elevated 
+privileges.
+
+```
+apiVersion: v1
+kind: Pod
+metadata:
+  name: busybox-kata
+spec:
+  runtimeClassName: kata-qemu
+  securityContext:
+    sysctls:
+    - name: kernel.shm_rmid_forced
+      value: "0"
+  containers:
+  - name: busybox-container
+    securityContext:
+      privileged: true
+    image: debian
+    command:
+        - sleep
+        - "3000"
+  initContainers:
+  - name: init-sys
+    securityContext:
+      privileged: true
+    image: busybox
+    command: ['sh', '-c', 'echo "64000" > /proc/sys/vm/max_map_count']
+```