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

On 01/24/2012 09:56 AM, Avi Kivity wrote:
On 01/24/2012 03:14 PM, Anthony Liguori wrote:
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.

We'll also have a hotpluggable variant which talks to some GPIOs.  A
motherboard may support multiple slots with such devices.

Yup. And boards can subclass the Ram device in order to be able to restrict the valid RAM sizes.

-m just becomes an analysis to set the ram size for a ram device. The infrastructure around RAMBlock, etc. would go away as each RAMBlock would correspond to a Ram device.

A video card would have a Ram device via composition.

IMO, overkill.

I think it's a lot of refactoring to get there and it's not the most critical infrastructure changes we have coming up, but I think it would be nice to get there eventually.

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.

Doesn't depth first make more sense?

Yes, it would be depth first, but you're looking for a preorder and post order hook. In otherwords:

void do_reset(DeviceState *dev)
    dev->pre_reset(dev);  // this is preorder
    dev->post_reset(dev);  // this is postorder

The PCI bus overloads the reset handler and does a post-order reset. I guess I can rationalize it after all but it's not entirely clear to me that a pre_reset hook isn't necessary too.

 A bus is considered reset after
all devices in the bus have been reset.

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

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?

Well, the actual bits in memory are state.  All other attributes are
indeed derived.

I just think that making any device that has a bit of RAM a composed
device is overkill.  What do we gain from it?  The cost is not trivial.

The cost of composition is pretty trivial.

struct VGADevice {
   DeviceState parent;

   Ram vga_ram;

static void vga_device_initfn(Object *obj)
   VGADevice *vga = VGA(obj);

   object_property_add_child(obj, "vram", (Object *)&vga_ram,
                             TYPE_RAM, NULL);

Now a user can control the VGA ram size by accessing vga/vram.size = 16M.


Anthony Liguori

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