[coreboot] how to implement variable read/write variable in coreboot

[Nico Huber]

On 30.09.2017 19:28, Peter Stuge wrote:
> Nico Huber wrote:
>>> We already have a simple coreboot-native key-value store: CBFS

Well, I had a look at it now. And indeed it is a coreboot-native key-
value store and could be used in the same way I imagined my proposed
storage format. But it's not simple.

>>
>> Well, I never looked close, but everybody kept telling me for years that
>> CBFS is not designed for updates.
> 
> But it is!?
Could be.

> 
> Not in-place updates, because CBFS isn't tied to eraseblock sizes,
> but a CBFS entry could be overwritten with 0x00, which makes readers
> scan for the next entry.

We have CBFS_COMPONENT_DELETED (0) type. It basically already works like
that.

> 
> If we haven't already we can declare entries with zero length names
> as deleted. Then the length remains, and no scan is neccessary. That
> would keep optimal read speed.

You are implying that read speed is already optimal?

What is really bothering me is that CBFS does two layers at once (key-
value store and attribution), and it has to for its current purpose:
CBFS can be sparse and has to be to allow entries at specific offsets
or with specific alignment. We probably don't want to mix that with
erasures. So I suppose we would want another FMAP entry for the era-
sable key-value store anyway and its unlikely that we'd need any of
the fancy CBFS features there. But if we account for full CBFS com-
patibility in the defragmentation scheme (or whatever we'll call it),
I fear, things will get way more complex then they need to be.

Nico