Re: [PATCH v1 3/6] MdeModulePkg: Map persistent (NV) memory

[Laszlo Ersek <lersek@redhat.com>]

(+Grant)

On 10/12/18 16:40, Sami Mujawar wrote:
> Some platforms are able to preserve a memory range across
> system resets. This memory can be used for the Non-Volatile
> variables storage. The PcdEmuVariableNvStoreReserved is
> used to specify this option.

This is the wrong approach, for two reasons.

(1) At a technical level, suck hacks are usually implemented by provding
a platform-specific FVB (firmware volume block) protocol / driver,
underneath the normal FTW and Variable drivers. The FVB implementation
can (technically) fake a "persistent" flash device in RAM that is
actually volatile.

(2) At a design level, this is an extremely bad idea though. Such
variables are only halfway non-volatile, and that fact always comes back
to bite users, sooner or later. I speak from experience with OVMF, but
more recently, there has been discussion on the USWG list too, about
platforms that can only fake non-volatility. I can't provide more
details on this open list about that, so I'll just give you a
Message-Id, and a Mantis ticket.

<a9a12a41-a1b8-3d86-5fc1-547abcff0a35@arm.com>

https://mantis.uefi.org/mantis/view.php?id=1961

Thanks,
Laszlo