docs: Add documentation for CPU parameter

This new document describes the available options for the parameter
`--cpus`, how to use them and what is their usage.

Fixes #3317

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

docs/cpu.md

@@ -0,0 +1,189 @@
+# CPU
+
+Cloud Hypervisor has many options when it comes to the creation of virtual
+CPUs. This document aims to explain what Cloud Hypervisor is capable of and
+how it can be used to meet the needs of very different use cases.
+
+## Options
+
+`CpusConfig` or what is known as `--cpus` from the CLI perspective is the way
+to set vCPUs options for Cloud Hypervisor.
+
+```rust
+struct CpusConfig {
+    boot_vcpus: u8,
+    max_vcpus: u8,
+    topology: Option<CpuTopology>,
+    kvm_hyperv: bool,
+    max_phys_bits: u8,
+    affinity: Option<Vec<CpuAffinity>>,
+}
+```
+
+```
+--cpus boot=<boot_vcpus>,max=<max_vcpus>,topology=<threads_per_core>:<cores_per_die>:<dies_per_package>:<packages>,kvm_hyperv=on|off,max_phys_bits=<maximum_number_of_physical_bits>,affinity=<list_of_vcpus_with_their_associated_cpuset>
+```
+
+### `boot`
+
+Number of vCPUs present at boot time.
+
+This option allows to define a specific number of vCPUs to be present at the
+time the VM is started. This option is mandatory when using the `--cpus`
+parameter. If `--cpus` is not specified, this option takes the default value
+of `1`, starting the VM with a single vCPU.
+
+Value is an unsigned integer of 8 bits.
+
+_Example_
+
+```
+--cpus boot=2
+```
+
+### `max`
+
+Maximum number of vCPUs.
+
+This option defines the maximum number of vCPUs that can be assigned to the VM.
+In particular, this option is used when looking for CPU hotplug as it lets the
+provide an indication about how many vCPUs might be needed later during the
+runtime of the VM.
+For instance, if booting the VM with 2 vCPUs and a maximum of 6 vCPUs, it means
+up to 4 vCPUs can be added later at runtime by resizing the VM.
+
+The value must be greater than or equal to the number of boot vCPUs.
+The value is an unsigned integer of 8 bits.
+
+By default this option takes the value of `boot`, meaning vCPU hotplug is not
+expected and can't be performed.
+
+_Example_
+
+```
+--cpus max=3
+```
+
+### `topology`
+
+Topology of the guest platform.
+
+This option gives the user a way to describe the exact topology that should be
+exposed to the guest. It can be useful to describe to the guest the same
+topology found on the host as it allows for proper usage of the resources and
+is a way to achieve better performances.
+
+The topology is described through the following structure:
+
+```rust
+struct CpuTopology {
+    threads_per_core: u8,
+    cores_per_die: u8,
+    dies_per_package: u8,
+    packages: u8,
+}
+```
+
+or the following syntax through the CLI:
+
+```
+topology=<threads_per_core>:<cores_per_die>:<dies_per_package>:<packages>
+```
+
+By default the topology will be `1:1:1:1`.
+
+_Example_
+
+```
+--cpus boot=2,topology=1:1:2:1
+```
+
+### `kvm_hyperv`
+
+Enable KVM Hyper-V emulation.
+
+When turned on, this option relies on KVM to emulate the synthetic interrupt
+controller (SynIC) along with synthetic timers expected by a Windows guest.
+A Windows guest usually runs on top of Microsoft Hyper-V, therefore expects
+these synthetic devices to be present. That's why KVM provides a way to emulate
+them and avoids failures running a Windows guest with Cloud Hypervisor.
+
+By default this option is turned off.
+
+_Example_
+
+```
+--cpus kvm_hyperv=on
+```
+
+### `max_phys_bits`
+
+Maximum size for guest's addressable space.
+
+This option defines the maximum number of physical bits for all vCPUs, which
+sets a limit for the size of the guest's addressable space. This is mainly
+useful for debug purpose.
+
+The value is an unsigned integer of 8 bits.
+
+_Example_
+
+```
+--cpus max_phys_bits=40
+```
+
+### `affinity`
+
+Affinity of each vCPU.
+
+This option gives the user a way to provide the host CPU set associated with
+each vCPU. It is useful for achieving CPU pinning, ensuring multiple VMs won't
+affect the performance of each other. It might also be used in the context of
+NUMA as it is way of making sure the VM can run on a specific host NUMA node.
+In general, this option is used to increase the performances of a VM depending
+on the host platform and the type of workload running in the guest.
+
+The affinity is described through the following structure:
+
+```rust
+struct CpuAffinity {
+    vcpu: u8,
+    host_cpus: Vec<u8>,
+}
+```
+
+or the following syntax through the CLI:
+
+```
+affinity=[<vcpu_id1>@[<host_cpu_id1>, <host_cpu_id2>], <vcpu_id2>@[<host_cpu_id3>, <host_cpu_id4>]]
+```
+
+The outer brackets define the list of vCPUs. And for each vCPU, the inner
+brackets attached to `@` define the list of host CPUs the vCPU is allowed to
+run onto.
+
+Multiple values can be provided to define each list. Each value is an unsigned
+integer of 8 bits.
+
+For instance, if one needs to run vCPU 0 on host CPUs from 0 to 4, the syntax
+using `-` will help define a contiguous range with `affinity=0@[0-4]`. The
+same example could also be described with `affinity=0@[0,1,2,3,4]`.
+
+A combination of both `-` and `,` separators is useful when one might need to
+describe a list containing host CPUs from 0 to 99 and the host CPU 255, as it
+could simply be described with `affinity=0@[0-99,255]`.
+
+As soon as one tries to describe a list of values, `[` and `]` must be used to
+demarcate the list.
+
+By default each vCPU runs on the entire host CPU set.
+
+_Example_
+
+```
+--cpus boot=3,affinity=[0@[2,3],1@[0,1]]
+```
+
+In this example, assuming the host has 4 CPUs, vCPU 0 will run exclusively on
+host CPUs 2 and 3, while vCPU 1 will run exclusively on host CPUs 0 and 1.
+Because nothing is defined for vCPU 2, it can run on any of the 4 host CPUs.