Re: [PATCH] kvm guest balloon driver

[Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>]

Avi Kivity wrote:
> Anthony Liguori wrote:
>> BTW, I don't think the target should be a config value.  You don't 
>> gain anything from it being in the config space and it's somewhat 
>> unnatural for a virtio device.  It makes more sense as a message to 
>> the guest.
>>   
>
> I disagree. The target is state, not an individual item that needs to 
> be acted on. Having it as a single variable means that multiple 
> changes are collapsed automatically, and that the the setting survives 
> module reload.
>
> It's like a volume control, it doesn't send messages when you turn it, 
> it just sets a value.

It's a value that is meant to be acted upon though, not something to be 
polled.  You want to tell the driver that it should now try to balloon 
to a certain value.  Or maybe not.  Maybe the driver should read the 
target from the config space but then have a "kick" message that tells 
it, hey, something's probably changed.

> (maybe a thermostat knob is a better analogy, with the driver being 
> the circuitry around the knob that tries to control the temperature to 
> match the setting)

FWIW, I'm pretty sure that most modern volume controls are actually 
button presses for either direction instead of a variable resistor but 
point taken ;-)

> I believe state-like controls will be useful for other settings, like 
> ethernet link state.

It's difficult with the current config interface since it treats 
everything as a discrete blob that is updated all at once.  I'm not 
really sure how to proceed.

Rusty: do you have an opinion here?

Regards,

Anthony Liguori
 

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