[edk2-devel] OVMF: NV Variable Store Layout of Larger Build Targets

[Devon Bautista]

Hi Gerd,

>> The current maximum image size of an OVMF image is 4MB, which is
>> insufficient for storing even a minimal and compressed kernel and initramfs.
>> To get around this, we've been maintaining our own fork of EDK2 that adds
>> 8MiB and 16MiB OVMF build targets that have enough room in the DXE volume to
>> store a reasonably-sized kernel and initramfs. However, it would be
>> convenient if upstream EDK2 supported these larger OVMF targets.
> I'm wondering whenever it makes sense to have the 8M option.  I think
> I'd tend to go straight to 16M (which is the max size we can do on x86).

On the Linuxboot side, we really only need 16MiB. However, I think
Laszlo justified an 8MiB target because the QEMU commit he pointed to
(referenced in my initial post) increased the absolute firmware size
limit to be 16MiB when setting the maximum (`pcms->max_fw_size`) in
`pc_machine_set_max_fw_size()`, but the default maximum if not set is 8MiB.

So I understand why an 8MiB target is justified, but, like you, I am not
sure if it's really needed.

>> However, as Laszlo mentioned, introducing a larger volume size is
>> compatibility breaking, and so seizing the opportunity to come up
>> with a larger non-volatile variable store layout is necessary.
>>
>> That said, I would like to use this thread to discuss among hardware
>> vendors an optimal variable store layout for these larger image sizes.
> The 2M -> 4M switch happened because the varstore was too small.  It was
> Confirm64KilobytesOfUnauthenticatedVariableStorage test of the the
> Microsoft Hardware Certification failing.  I guess Microsoft has good
> reasons to test for 64k varstore, probably they expect this is big
> enough in practice.
>
> The varstore size of the 4M layout is *way* above that (see 2M -> 4M
> commit message):
>
>   Variable store      56 ->   256 ( +200)
>   Spare area          64 ->   264 ( +200)
>
> Assuming 256k varstore is more than enough:  Sticking to the 4M variable
> store layout for the 16M (and maybe 8M) builds looks like the best
> option to me.  I think the varstore would be identical for 4M and 16M
> builds then, so it should be possible to switch guests from 4M to 16M
> while keeping the varstore.

Keeping the 4MiB varstore layout would be the most compatible and
straightforward option and is what I would want to go with.

But I also think that it might make sense when introducing a
considerably larger build target to account for any possible increases
in variable store size that vendors may expect in the future. I for one
dismay any further size increase, but I suppose the more relevant
question is, is 256KiB of varstore enough for vendors?

-- 
Best,
Devon


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