I remember ARM platform could have a PEI-less design so that SEC directly invokes DXE.

So I can imagine that a SEC logic to create the VARIABLE_RUNTIME_CACHE_INFO HOB.

Then it comes to how to calculate the size before bios boots.
I think it's doable.
There are 3 caches. Volatile cache size is hardcode by a PCD. NV cache size can equal to the bios NV variable region size. HOB cache size can be calculated by:
  1. collect all default values in IFR/VFR
  2. convert the default variable to auth/non-auth type depending on the BIOS NV variable region format.
Both steps can be performed in bios-build phase. There is no runtime information needed.

Thanks,
Ray
From: Nhi Pham <nhi@os.amperecomputing.com>
Sent: Monday, May 20, 2024 9:01
To: devel@edk2.groups.io <devel@edk2.groups.io>; Tan, Dun <dun.tan@intel.com>
Cc: Ni, Ray <ray.ni@intel.com>; Liming Gao <gaoliming@byosoft.com.cn>; Wu, Jiaxin <jiaxin.wu@intel.com>; Ard Biesheuvel <ardb+tianocore@kernel.org>; Leif Lindholm <quic_llindhol@quicinc.com>; Sami Mujawar <sami.mujawar@arm.com>; Gerd Hoffmann <kraxel@redhat.com>; Andrew Fish <afish@apple.com>; Yao, Jiewen <jiewen.yao@intel.com>
Subject: Re: [edk2-devel] [PATCH 0/9] Allocate and unblock variable runtime cache buffer in PEI
 
On 5/17/2024 4:49 PM, duntan via groups.io wrote:
> This patch set defines a new VARIABLE_RUNTIME_CACHE_INFO HOB. The HOB is used to store the address and size of the buffer that will be used for variable runtime service when the PcdEnableVariableRuntimeCache is TRUE.
> In following patches, when PcdEnableVariableRuntimeCache is TRUE, VariablePei will install a callback of gEfiPeiMemoryDiscoveredPpiGuid to allocate the needed buffer for different type variable runtime cache and build the HOB.
> Then VariableSmmRuntimeDxe driver will consume gEdkiiVariableRuntimeCacheInfoHobGuid to initialize the variable runtime cache related content. The code to allocate and unblock the runtime cache buffer in VariableSmmRuntimeDxe is also removed in this patc set.
>
> PR for review: https://github.com/tianocore/edk2/pull/5607

Per design, SMM or StandaloneMM needs to access these runtime cache
buffers for cache coherency. I'm not sure how to implement the
MmUnblockMemoryLib for ARM to dynamically request mapping of the
non-secure runtime cache buffers in StandaloneMM (Secure World). Is it
possible to have these runtime buffers allocated statically with
predefined PCD at build time. On ARM, they can also define the buffers
in device tree (manifest)?

Thanks,
Nhi
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