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Re: [Qemu-devel] Re: virtio: Add memory statistics reporting to the ball

From: Anthony Liguori
Subject: Re: [Qemu-devel] Re: virtio: Add memory statistics reporting to the balloon driver
Date: Tue, 10 Nov 2009 08:36:14 -0600
User-agent: Thunderbird (X11/20090825)

Rusty Russell wrote:
On Tue, 10 Nov 2009 03:02:06 am Adam Litke wrote:
A simpler approach is to collect memory statistics in the virtio
balloon driver and communicate them to the host via the device config space.

There are two issues I see with this.  First, there's an atomicity problem
since you can't tell when the stats are consistent.

Actually, config writes always require notification from the guest to the host. This means the host knows when they config space is changed so atomicity isn't a problem.

In fact, if it were a problem, then the balloon driver would be fundamentally broken because target and actual are stored in the config space.

If you recall, we had this discussion originally wrt the balloon driver :-)

  Second, polling is

As opposed to? The guest could set a timer and update the values periodically but that's even uglier because then the host cannot determine the update granularity.

A stats vq might solve this more cleanly?

actual and target are both really just stats. Had we implemented those with a vq, I'd be inclined to agree with you but since they're implemented in the config space, it seems natural to extend the config space with other stats.


Anthony Liguori

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