Re: [PATCH v2 3/8] OvmfPkg: conditionally disable reserved memory varstore emulation at build

[Laszlo Ersek <lersek@redhat.com>]

On 12/01/17 09:51, Ard Biesheuvel wrote:
> On 30 November 2017 at 16:30, Laszlo Ersek <lersek@redhat.com> wrote:
>> (All of the below is only relevant for SMM_REQUIRE=FALSE.)
>>
>> For the emulated variable store, PlatformPei allocates reserved memory (as
>> early as possible, so that the address remains the same during reboot),
>> and PcdEmuVariableNvStoreReserved carries the address to
>> EmuVariableFvbRuntimeDxe.
>>
>> In addition, QemuFlashFvbServicesRuntimeDxe is always launched before
>> EmuVariableFvbRuntimeDxe, so that if flash variables are available,
>> QemuFlashFvbServicesRuntimeDxe can set PcdFlashNvStorageVariableBase64
>> first, and EmuVariableFvbRuntimeDxe can exit early. This ordering is
>> currently enforced by adding QemuFlashFvbServicesRuntimeDxe to the APRIORI
>> DXE file.
>>
>> All of this is unnecessary when MEM_VARSTORE_EMU_ENABLE is set to FALSE.
>> In such a build,
>>
>> - (almost) remove the dynamic default for PcdEmuVariableNvStoreReserved
>>   (we can't really do this because the PcdSet64() in
>>   ReserveEmuVariableNvStore() wouldn't compile),
>>
> 
> If that is the only concern, and the value is irrelevant, you could
> make it a patchable PCD instead

This sounds interesting; I've never used patchable PCDs. Can you please
elaborate?

Do you mean that for (SMM_REQUIRE==TRUE ||
MEM_VARSTORE_EMU_ENABLE==FALSE), the DSC file should list
PcdEmuVariableNvStoreReserved somewhere else?

Does PcdSet work on patchable PCDs?

Thanks!
Laszlo