# QEMU/Guest Firmware Interface for AMD SEV and SEV-ES

## Overview

The guest firmware image (OVMF) may contain some configuration entries which are used by QEMU before the guest launches. These are listed in a GUIDed table at a known location in the firmware image. QEMU parses this table when it loads the firmware image into memory, and then QEMU reads individual entries when their values are needed.

Though nothing in the table structure is SEV-specific, currently all the entries in the table are related to SEV and SEV-ES features.

### Table parsing in QEMU

The table is parsed from the footer: first the presence of the table footer GUID (96b582de-1fb2-45f7-baea-a366c55a082d) at 0xffffffd0 is verified. If that is found, two bytes at 0xffffffce are the entire table length.

Then the table is scanned backwards looking for the specific entry GUID.

QEMU files related to parsing and scanning the OVMF table:

  - `hw/i386/pc_sysfw_ovmf.c`

The edk2 firmware code that constructs this structure is in the OVMF Reset Vector file.

### Table memory layout

GPA | Length | Description
--- | --- | ---
0xffffff80 | 4 | Zero padding
0xffffff84 | 4 | SEV hashes table base address
0xffffff88 | 4 | SEV hashes table size (=0x400)
0xffffff8c | 2 | SEV hashes table entry length (=0x1a)
0xffffff8e | 16 | SEV hashes table GUID: 7255371f-3a3b-4b04-927b-1da6efa8d454
0xffffff9e | 4 | SEV secret block base address
0xffffffa2 | 4 | SEV secret block size (=0xc00)
0xffffffa6 | 2 | SEV secret block entry length (=0x1a)
0xffffffa8 | 16 | SEV secret block GUID: 4c2eb361-7d9b-4cc3-8081-127c90d3d294
0xffffffb8 | 4 | SEV-ES AP reset RIP
0xffffffbc | 2 | SEV-ES reset block entry length (=0x16)
0xffffffbe | 16 | SEV-ES reset block entry GUID: 00f771de-1a7e-4fcb-890e-68c77e2fb44e
0xffffffce | 2 | Length of entire table including table footer GUID and length (=0x72)
0xffffffd0 | 16 | OVMF GUIDed table footer GUID: 96b582de-1fb2-45f7-baea-a366c55a082d
0xffffffe0 | 8 | Application processor entry point code
0xffffffe8 | 8 | "0000VTF0"
0xfffffff0 | 16 | Reset vector code

## Table entries description

### SEV-ES reset block

Entry GUID: 00f771de-1a7e-4fcb-890e-68c77e2fb44e

For the initial boot of an AP under SEV-ES, the "reset" RIP must be programmed to the RAM area defined by this entry. The entry's format is:

- IP value [0:15]

- CS segment base [31:16]

A hypervisor reads the CS segment base and IP value. The CS segment base value represents the high order 16-bits of the CS segment base, so the hypervisor must left shift the value of the CS segment base by 16 bits to form the full CS segment base for the CS segment register. It would then program the EIP register with the IP value as read.

### SEV secret block

Entry GUID: 4c2eb361-7d9b-4cc3-8081-127c90d3d294

This describes the guest RAM area where the hypervisor should inject the Guest Owner secret (using SEV_LAUNCH_SECRET).

### SEV hashes table

Entry GUID: 7255371f-3a3b-4b04-927b-1da6efa8d454

This describes the guest RAM area where the hypervisor should install a table describing the hashes of certain firmware configuration device files that would otherwise be passed in unchecked. The current use is for the kernel, initrd and command line values, but others may be added.