[edk2-rfc] [edk2-devel] RFC: design review for TDVF in OVMF

[Laszlo Ersek]

On 06/08/21 14:27, Xu, Min M wrote:
> On 06/04/2021 12:12 AM, Laszlo wrote:

>> But it counts as an absolute disaster nowadays, and should not be revived in
>> any platform. If you don't have pflash in TDX guests, just accept that you
>> won't have non-volatile variables. And link PlatformFvbLibNull into
>> EmuVariableFvbRuntimeDxe. You're going to need a separate
>> PlatformBootManagerLib instance anyway.
>>
> I have a question here, that if PlatformFvbLibNull is linked into EmuVaiableFvRuntimeDxe,
> Does it mean it cannot write to the in-RAM variable store?

No, that's not the case; PlatformFvbLibNull only turns the hooks
(special callbacks) into no-ops; the normal operation of
EmuVariableFvbRuntimeDxe is not disrupted.

The APIs in the PlatformFvbLib class are PlatformFvbDataRead,
PlatformFvbDataWritten, PlatformFvbBlocksErased; they are called at the
ends of the functions FvbProtocolEraseBlocks(), FvbProtocolWrite(),
FvbProtocolRead(), in "OvmfPkg/EmuVariableFvbRuntimeDxe/Fvb.c". If the
PlatformFvb* APIs do nothing, that's not a problem for
EmuVariableFvbRuntimeDxe.

(In fact, even in the non-Null instance -- that is, in the
EmuVariableFvbLib instance --, the read and erase callbacks are empty;
and the write callback only signals an event, at best.)

>> (We should have removed EmuVariableFvbRuntimeDxe a long time ago from
>> the traditional OVMF platforms, i.e. made pflash a hard requirement, even
>> when SMM is not built into the platform -- but whenever I tried that, Jordan
>> always shot me down.)
>>
> I am afraid in TDVF we have to use EmuVariableFvRuntimeDxe to emulate the
> in-RAM, as I explained pflash is not part of the *board* in TDX setup.

Using EmuVariableFvbRuntimeDxe in the TDVF platform is fine (with the
Null hooks, see above), as long as we carefully document the expected /
resultant behavior of the UEFI variable services.

(This is not a comment on my part on the SB situation, which remains an
open question for TDVF, for the time being.)

Thanks
Laszlo



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