uefi mm protocol draft

  Hi folks,

Here is a spec draft for the uefi mm protocol.
See also https://github.com/coconut-svsm/svsm/pull/694

take care,
  Gerd

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## SVSM UEFI MM PROTOCOL

Process MM communication requests from edk2 firmware.

Usually this request serialization format is used by edk2 for
communication between normal mode and management mode (MM for short).

This SVSM protocol is a thin wrapper to allow edk2 firmware send those
requests to the SVSM instead.  This allows the SVSM provide services
which are usually running in SMM mode on bare metal or
non-confidential VMs, specifically an UEFI variable store.

### protocol number

```
pub const SVSM_UEFI_MM_PROTOCOL: u32 = 4;
```
preliminary, not yet officially assigned

### query command

```
const SVSM_UEFI_MM_QUERY: u32 = 1;
```

Returns success in case the protocol is supported by the svsm.

Return parameters:

 * RCX: protocol feature flags.  There are no features defined yet.
   All bits are reserved and must be zero.

### setup command

```
const SVSM_UEFI_MM_SETUP: u32 = 2;
```

Setup buffer for messages.

Parameters:
 * RCX: buffer address (guest physical).
 * RDX: buffer size

### reset command

```
const SVSM_UEFI_MM_RESET: u32 = 3;
```

Reset protocol (communication buffer).

### request command

```
const SVSM_UEFI_MM_REQUEST: u32 = 4;
```

Process request in the communication buffer, store response message in
the buffer.

### request format

The request serialization format is specified by edk2, the short
overview below is purely informational.

The buffer starts with this header:

```
// EFI_MM_COMMUNICATE_HEADER
#[repr(C, packed)]
pub struct MmCoreHeader {
    pub guid: [u8; 16],
    pub size: u64,
}
```

The `guid` specifies the receiver of the message (typically an efi
protocol guid or an efi event guid), and therefore also the format of
the request data following this header.  The `size` field specified
the size of the request data (excluding the header).

### edk2 references

 * MdePkg/Include/Protocol/MmCommunication.h
 * MdeModulePkg/Include/Guid/SmmVariableCommon.h
 * MdeModulePkg/Include/Guid/VarCheckPolicyMmi.h

### linux refernces

The linux kernel has an efi variable driver implementation using this
request format in `drivers/firmware/efi/stmm`.  It sends those
requests to the edk2 MM code running in arm secure partition.