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| -rw-r--r-- | docs/specs/index.rst | 1 | ||||
| -rw-r--r-- | docs/specs/sev-guest-firmware.rst | 125 |
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diff --git a/docs/specs/index.rst b/docs/specs/index.rst index ecc4389..2a35700 100644 --- a/docs/specs/index.rst +++ b/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 diff --git a/docs/specs/sev-guest-firmware.rst b/docs/specs/sev-guest-firmware.rst new file mode 100644 index 0000000..3f7f082 --- /dev/null +++ b/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 |