Re: [Qemu-devel] [RFC] block: Removed coroutine ownership assumption

[Peter Maydell]

On 2 July 2012 11:44, Kevin Wolf <address@hidden> wrote:
> Am 02.07.2012 12:18, schrieb Peter Maydell:
>> Why complicate things by adding code for "if this is the first
>> access then read in the file"?
>
> Because then it works. :-)
>
> Migration works more or less like this:
>
> 1. Destination creates device model based on command line
> 2. RAM is copied live, source keeps running
> 3. Source stops, device state is transferred
> 4. Destination starts running the VM
>
> Reading from a block device is meaningful the earliest in step 3,
> because at earlier points the guest still runs on the source and can
> overwrite the data on the block device. If you're reading in the whole
> image, you're doing it in step 1, so your data will be outdated by the
> time the migration completes.

For pflash_cfi01 migration will work because although we read
the file (slightly pointlessly) in step 1, we will get the correct
contents transferred over in step 2, because we call vmstate_register_ram()
in device init.

We need to migrate flash contents like that anyway, to handle the
case of "no backing file, flash starts empty and gets whatever the
guest writes to it for as long as the guest is alive".

> The approach with migrating the block device content probably works for
> your 64k flash, but what about my hard disk?

I'm not claiming this is a good approach for everything, just for
some things.

-- PMM