Re: [Qemu-devel] [qemu PATCH] hw/i386/pc_sysfw: support more than one flash drive

[Markus Armbruster]

Laszlo Ersek <address@hidden> writes:

> On 11/27/13 18:22, Markus Armbruster wrote:
>
>> Perhaps the proper way to back partially writable flash contents isn't
>> splitting it into two devices, but backing a single device with a COW.
>> The backing file has initial contents (say BIOS image), the delta may
>> have additional contents (say non-volatile configuration), and picks up
>> whatever the device model permits to be written to the flash.
>
> Kevin brought this up before. The problem is that this prevents you from
> upgrading the read-only part in O(1).

Err, what's wrong with rebasing the COW to a new backing file?

Oh, I see you considered it:

> (Unless of course you're willing to make the backing file raw *and* to
> poke directly in it when you upgrade the bios binary part. Theoretically

No, don't update the backing file, *replace* it.

If the device model permitted a write to a cluster within the BIOS part,
the replacement won't be visible, so don't do that :)

> you could do this, because the divide between the varstore and the
> binary part at 128KB falls at a qcow2 block boundary (supposing a 64K
> qcow2 block size), but in practice this is a horrible idea :))

Yes, with COW the COW clustering apply to the division between BIOS part
and non-volatile configuration part.

> Let's not veer off into architecting a new star destroyer. I think I
> have plenty ammo from your great notes thus far that I can hack up a v2
> with :)

Hmmmm, pretty star destroyers...