docs: update virt/vm

virt/vm.md

@@ -2,7 +2,7 @@
 title: Machine definition
 description: Virtual machine hardware
 published: true
-date: 2025-06-01T17:41:22.711Z
+date: 2025-06-02T17:41:18.805Z
 tags: 
 editor: markdown
 dateCreated: 2025-06-01T17:37:29.262Z
@@ -12,7 +12,29 @@ dateCreated: 2025-06-01T17:37:29.262Z
 
 Libvirt uses XML files to define virtual machine hardware.
 
-In the context of libvirt, a virtual machine or guest system is called a domain. In turn, a domain type refers to the hypervisor used for running the virtual machine.
+In the context of libvirt, a virtual machine or guest system is called a domain. 
+
+A domain *type* refers to the hypervisor used for running the virtual machine.
+
+- Truncated snippet of an imaginary virtual machine called *phyllome*:
+
+```
+<domain type='kvm'>
+  <name>phyllome</name>
+  <memory unit="GiB">1</memory>
+  <vcpu placement='static'>1</vcpu>
+  <os firmware="efi">
+  <devices>
+    <emulator>/usr/bin/qemu-system-x86_64</emulator>
+    <input type='keyboard' bus='virtio'>
+    </input>
+	</devices>
+</domain>
+```
+
+One can identify the domain type, KVM, the machine name, *phyllome*, the allocated memory, 1 GiB, the vCPU allocation, 1vCPU, the firmware type, UEFI. 
+
+These elements are primarily managed by Linux and KVM, whereas the elements within the devices section are managed by QEMU. One can find the keyboard, for instance.
 
 ## Domain type
 
@@ -24,8 +46,7 @@ At the moment, Phyllome OS focuses solely on hardware-assisted virtualization wi
 </domain>
 ```
 
-Not all QEMU commands are supported by Libvirt. To [pass QEMU commands](https://www.libvirt.org/drvqemu.html#pass-through-of-arbitrary-qemu-commands
+Not all QEMU commands are supported by Libvirt. To [pass QEMU commands](https://www.libvirt.org/drvqemu.html#pass-through-of-arbitrary-qemu-commands), one need to use the following instead and add the QEMU command line at the end :
-), one need to use the following instead and add the QEMU command line at the end :
 
 ```
 <domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
@@ -40,12 +61,46 @@ Not all QEMU commands are supported by Libvirt. To [pass QEMU commands](https://
 </domain>
 ```
 
-> If QEMU command lines are not immediately provided, it will automatically revert to simply `<domain type='kvm'>` upon writing changes to the domain
+> If QEMU command lines are not immediately provided, it will automatically revert to simply `<domain type='kvm'>` when writing changes to the domain definition
 {.is-info}
 
-Under QEMU, the said domain definition translates to: 
+Under QEMU, the said domain definition translates simply to: 
 
 ```
 qemu-system-x86_64 -enable-kvm [...]
 ```
 
+## Memory
+
+There are many ways to allocate memory. For maximum performance, one could reserve memory to virtual machine in such a way that it may not be used by the host or any other virtual machine.
+
+A sane default, which does not maximize performance, is the following:
+
+```
+<domain type='kvm'>
+[...]
+	<memory unit="GiB">4</memory>
+  <currentMemory unit="GiB">1</currentMemory>
+[...]  
+	<devices>
+   	<memballoon model="virtio-non-transitional"/>
+  </devices>
+</domain>
+```
+
+Up to 4 GiB of memory is allocated to the virtual machine, with a minimum of 1 GiB. 
+
+A memballon device is defined, which can release memory to the host and other virtual machine in case it is not needed anymore.
+
+```
+<domain type='kvm'>
+[...]
+  <memoryBacking>
+   	<source type="memfd"/>
+   	<access mode="shared"/>
+  </memoryBacking>
+[...]
+</domain>
+```
+
+What is also done is to mark the memory as shared memory, which is necessary for other features to work, like *virtiofs*.