Re: [PATCH v2 05/13] conf: Introduce virtio-mem <memory/> model

The virtio-mem is paravirtualized mechanism of adding/removing memory to/from a VM. A virtio-mem-pci device is split into blocks of equal size which are then exposed (all or only a requested portion of them) to the guest kernel to use as regular memory. Therefore, the device has two important attributes: 1) block-size, which defines the size of a block 2) requested-size, which defines how much memory (in bytes) is the device requested to expose to the guest. The 'block-size' is configured on command line and immutable throughout device's lifetime. The 'requested-size' can be set on the command line too, but also is adjustable via monitor. In fact, that is how management software places its requests to change the memory allocation. If it wants to give more memory to the guest it changes 'requested-size' to a bigger value, and if it wants to shrink guest memory it changes the 'requested-size' to a smaller value. Note, value of zero means that guest should release all memory offered by the device. Of course, guest has to cooperate. Therefore, there is a third attribute 'size' which is read only and reflects how much memory the guest still has. This can be different to 'requested-size', obviously. Because of name clash, I've named it 'actualsize' and it is dealt with in future commits (it is a runtime information anyway). In the backend, memory for virtio-mem is backed by usual objects: memory-backend-{ram,file,memfd} and their size puts the cap on the amount of memory that a virtio-mem device can offer to a guest. But we are already able to express this info using <size/> under <target/>. Therefore, we need only two more elements to cover 'block-size' and 'requested-size' attributes. This is the XML I've came up with: <memory model='virtio-mem'> <source> <nodemask>1-3</nodemask> <pagesize unit='KiB'>2048</pagesize> </source> <target> <size unit='KiB'>2097152</size> <node>0</node> <block unit='KiB'>2048</block> <requested unit='KiB'>1048576</requested> </target> <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/> </memory> I hope by now it is obvious that: 1) 'requested-size' must be an integer multiple of 'block-size', and 2) virtio-mem-pci device goes onto PCI bus and thus needs PCI address. Then there is a limitation that the minimal 'block-size' is transparent huge page size (I'll leave this without explanation). Since now we have (possibly) two or more devices that allow memory inflation/deflation and accounting for all of them (and thus keeping <currentMemory/> updated) might be hard. Therefore, I'm deliberately forbidding memballoon. It's okay - virtio-mem is superior to memballoon anyway. We can always reevaluate later.