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

[Avi Kivity <avi@redhat.com>]

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?

> 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.

> But since some video cards can make use of dedicated external RAM, I
> don't think any video card really depends on initial RAM state.
>
> What's most likely here is that the VGA BIOS of a Cirrus card sets an
> initial RAM state during device initialization.

Yes, that's likely.  DRAMs aren't reset and some state survives even a
short power off.

>
> 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.

> But viewing RAM as just another device, having Xen only restore a
> subset of devices should be a reasonable thing to do moving forward. 
> The main problem here I believe is that we have part of the VGA Bios
> functionality in the hardware emulation.

Doesn't the main BIOS clear the screen first thing at boot?  Not even
sure the reset is needed.

-- 
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