docs: Add spec of OVMF GUIDed table for SEV guests

Add docs/specs/sev-guest-firmware.rst which describes the GUIDed table
in the end of OVMF's image which is parsed by QEMU, and currently used
to describe some values for SEV and SEV-ES guests.

Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220103091413.2869-1-dovmurik@linux.ibm.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

docs/specs/index.rst

@@ -18,3 +18,4 @@ guest hardware that is specific to QEMU.
    acpi_mem_hotplug
    acpi_pci_hotplug
    acpi_nvdimm
+   sev-guest-firmware

docs/specs/sev-guest-firmware.rst

@@ -0,0 +1,125 @@
+====================================================
+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      | "\0\0\0\0VTF\0"                         |
++------------+--------+-----------------------------------------+
+| 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.
+
+
+.. _OVMF Reset Vector file:
+   https://github.com/tianocore/edk2/blob/master/OvmfPkg/ResetVector/Ia16/ResetVectorVtf0.asm