[PULL 34/37] docs/specs: Add secure IPL documentation

[Eric Farman <farman@linux.ibm.com>]

From: Zhuoying Cai <zycai@linux.ibm.com>

Add documentation for secure IPL

Signed-off-by: Collin Walling <walling@linux.ibm.com>
Signed-off-by: Zhuoying Cai <zycai@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20260730214624.2328883-33-zycai@linux.ibm.com
Signed-off-by: Eric Farman <farman@linux.ibm.com>
---
 docs/specs/s390x-secure-ipl.rst | 55 +++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/docs/specs/s390x-secure-ipl.rst b/docs/specs/s390x-secure-ipl.rst
index 7a603eb956..82b21600af 100644
--- a/docs/specs/s390x-secure-ipl.rst
+++ b/docs/specs/s390x-secure-ipl.rst
@@ -1,5 +1,60 @@
 .. SPDX-License-Identifier: GPL-2.0-or-later
 
+s390 Secure IPL
+===============
+
+Secure IPL (a.k.a. secure boot) enables s390-ccw virtual machines to
+leverage qcrypto libraries and z/Architecture emulation to verify the
+integrity of signed kernels. The qcrypto libraries are used to perform
+certificate validation and signature-verification, whereas the
+z/Architecture emulation is used to ensure secure IPL data has not
+been tampered with, convey data between QEMU and guest code, and set up
+the relevant secure IPL data structures with verification results.
+
+To find out more about using this feature, see
+:doc:`documentation </system/s390x/secure-ipl>`.
+
+Note that "guest code" will refer to the s390-ccw BIOS unless stated
+otherwise.
+
+Both QEMU and guest code work in cooperation to perform secure IPL. The Secure
+Code Loading Attributes Facility (SCLAF) is used to check the Secure Code
+Loading Attribute Block (SCLAB) and ensure that secure IPL data has not
+been tampered with. DIAGNOSE 'X'320' is invoked by guest code to query
+the certificate store info and retrieve specific certificates from QEMU.
+DIAGNOSE 'X'508' is used by guest code to leverage qcrypto libraries to
+perform signature-verification in QEMU. Lastly, guest code generates and
+appends an IPL Information Report Block (IIRB) at the end of the IPL
+Parameter Block (IPLB), which is used by the kernel to store signed and
+verified entries.
+
+The logical steps are as follows:
+
+- guest code reads data payload from disk (e.g. stage3 boot loader, kernel)
+- guest code checks the validity of the SCLAB
+- guest code invokes DIAG 508 subcode 1 and provides the payload
+- QEMU handles DIAG 508 request by reading the payload and retrieving the
+  certificate store
+- QEMU DIAG 508 utilizes handler qcrypto libraries to perform
+  signature-verification on the payload, attempting with each cert in the store
+  (until success or exhausted)
+- QEMU DIAG 508 returns:
+
+  - success: index of cert used to verify payload
+  - failure: error code
+
+- guest code is expected to respond to this operation by:
+
+  - success: retrieves cert from store via DIAG 320 using returned index
+  - failure: reports with warning (audit mode), aborts with error (secure mode)
+
+- guest code appends IIRB at the end of the IPLB
+- guest code kicks off IPL
+
+More information regarding the respective DIAGNOSE commands and IPL data
+structures are outlined within this document.
+
+
 s390 Certificate Store and Functions
 ------------------------------------
 
-- 
2.55.0