Re: [Qemu-devel] [PATCH v4 0/6] save/restore on Xen

[Anthony Liguori]

On 01/24/2012 05:10 AM, Avi Kivity wrote:
> On 01/23/2012 07:18 PM, Anthony Liguori wrote:
> > Generally speaking, RAM is an independent device in most useful cases.
>
> Can you give examples?  Do you mean a subdevice with composition, or a
> really independent device?

I expect we'll have one Ram device.  It's size will be configurable.  One Ram 
device will hang off of the machine which would be the main ram.

A video card would have a Ram device via composition.

The important consideration about reset is how it propagates.  My expectation is 
that we'll propagate reset through the composition tree in a preorder 
transversal.  That means in the VGA device reset function, it's child device 
(Ram) has not been reset yet.

> > Onboard RAM is a very special case because it's extremely unusual.
>
> What's unusual (or even extremely unusual) about it?  I don't see a
> difference between a few billion bits of state and, say, the few bits of
> state in an RTC device.

Okay, but let's not get philosophical here :-)  There are very few devices that 
have a DRAM chip that is mapped through a bar into the main address space and 
behaves (usually) like normal RAM.

> > We really should view RAM as just another device so I don't like the
> > idea of propagating a global concept of "when RAM is restored" because
> > that treats it specially compared to other devices.
>
> Agree.  In fact the first step has been taken as now creating a RAM
> region with memory_region_init_ram() doesn't register it for migration.
> The next step would be a VMSTATE_RAM() to make it part of the containing
> device.

That's not necessary (or wise).

Let's not confuse a Ram device with a MemoryRegion.  They are separate things 
and should be treated as separate things.  I thought we discussed that 
MemoryRegions are stateless (or at least, there state is all derived) and don't 
need to be serialized?

Regards,

Anthony Liguori