126 lines
5.2 KiB
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126 lines
5.2 KiB
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====================================================
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QEMU/Guest Firmware Interface for AMD SEV and SEV-ES
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====================================================
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Overview
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========
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The guest firmware image (OVMF) may contain some configuration entries
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which are used by QEMU before the guest launches. These are listed in a
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GUIDed table at a known location in the firmware image. QEMU parses
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this table when it loads the firmware image into memory, and then QEMU
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reads individual entries when their values are needed.
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Though nothing in the table structure is SEV-specific, currently all the
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entries in the table are related to SEV and SEV-ES features.
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Table parsing in QEMU
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---------------------
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The table is parsed from the footer: first the presence of the table
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footer GUID (96b582de-1fb2-45f7-baea-a366c55a082d) at 0xffffffd0 is
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verified. If that is found, two bytes at 0xffffffce are the entire
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table length.
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Then the table is scanned backwards looking for the specific entry GUID.
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QEMU files related to parsing and scanning the OVMF table:
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- ``hw/i386/pc_sysfw_ovmf.c``
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The edk2 firmware code that constructs this structure is in the
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`OVMF Reset Vector file`_.
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Table memory layout
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-------------------
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+------------+--------+-----------------------------------------+
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| GPA | Length | Description |
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+============+========+=========================================+
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| 0xffffff80 | 4 | Zero padding |
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+------------+--------+-----------------------------------------+
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| 0xffffff84 | 4 | SEV hashes table base address |
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+------------+--------+-----------------------------------------+
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| 0xffffff88 | 4 | SEV hashes table size (=0x400) |
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+------------+--------+-----------------------------------------+
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| 0xffffff8c | 2 | SEV hashes table entry length (=0x1a) |
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+------------+--------+-----------------------------------------+
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| 0xffffff8e | 16 | SEV hashes table GUID: |
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| | | 7255371f-3a3b-4b04-927b-1da6efa8d454 |
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+------------+--------+-----------------------------------------+
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| 0xffffff9e | 4 | SEV secret block base address |
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+------------+--------+-----------------------------------------+
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| 0xffffffa2 | 4 | SEV secret block size (=0xc00) |
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+------------+--------+-----------------------------------------+
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| 0xffffffa6 | 2 | SEV secret block entry length (=0x1a) |
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+------------+--------+-----------------------------------------+
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| 0xffffffa8 | 16 | SEV secret block GUID: |
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| | | 4c2eb361-7d9b-4cc3-8081-127c90d3d294 |
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+------------+--------+-----------------------------------------+
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| 0xffffffb8 | 4 | SEV-ES AP reset RIP |
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+------------+--------+-----------------------------------------+
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| 0xffffffbc | 2 | SEV-ES reset block entry length (=0x16) |
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+------------+--------+-----------------------------------------+
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| 0xffffffbe | 16 | SEV-ES reset block entry GUID: |
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| | | 00f771de-1a7e-4fcb-890e-68c77e2fb44e |
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+------------+--------+-----------------------------------------+
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| 0xffffffce | 2 | Length of entire table including table |
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| | | footer GUID and length (=0x72) |
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+------------+--------+-----------------------------------------+
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| 0xffffffd0 | 16 | OVMF GUIDed table footer GUID: |
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| | | 96b582de-1fb2-45f7-baea-a366c55a082d |
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+------------+--------+-----------------------------------------+
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| 0xffffffe0 | 8 | Application processor entry point code |
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+------------+--------+-----------------------------------------+
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| 0xffffffe8 | 8 | "\0\0\0\0VTF\0" |
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+------------+--------+-----------------------------------------+
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| 0xfffffff0 | 16 | Reset vector code |
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+------------+--------+-----------------------------------------+
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Table entries description
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=========================
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SEV-ES reset block
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------------------
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Entry GUID: 00f771de-1a7e-4fcb-890e-68c77e2fb44e
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For the initial boot of an AP under SEV-ES, the "reset" RIP must be
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programmed to the RAM area defined by this entry. The entry's format
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is:
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* IP value [0:15]
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* CS segment base [31:16]
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A hypervisor reads the CS segment base and IP value. The CS segment
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base value represents the high order 16-bits of the CS segment base, so
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the hypervisor must left shift the value of the CS segment base by 16
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bits to form the full CS segment base for the CS segment register. It
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would then program the EIP register with the IP value as read.
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SEV secret block
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----------------
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Entry GUID: 4c2eb361-7d9b-4cc3-8081-127c90d3d294
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This describes the guest RAM area where the hypervisor should inject the
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Guest Owner secret (using SEV_LAUNCH_SECRET).
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SEV hashes table
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----------------
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Entry GUID: 7255371f-3a3b-4b04-927b-1da6efa8d454
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This describes the guest RAM area where the hypervisor should install a
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table describing the hashes of certain firmware configuration device
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files that would otherwise be passed in unchecked. The current use is
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for the kernel, initrd and command line values, but others may be added.
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.. _OVMF Reset Vector file:
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https://github.com/tianocore/edk2/blob/master/OvmfPkg/ResetVector/Ia16/ResetVectorVtf0.asm
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