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

[David Hildenbrand]

On 18.02.21 14:31, Michal Privoznik wrote:
> 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.

That's a bad idea. It'll still be used for getting memory stats, free 
page hinting and free page reporting.

Very weird use cases might even want to mix balloon inflation/deflation 
with virtio-mem ...

-- 
Thanks,

David / dhildenb