efi_loader: add a README.iscsi describing booting via iSCSI

The appended README explains how U-Boot and iPXE can be used
to boot a diskless system from an iSCSI SAN.

The maintainer for README.efi and README.iscsi is set.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
[agraf: s/Adress/Address/]
Signed-off-by: Alexander Graf <agraf@suse.de>

1 files changed, 159 insertions, 0 deletions

diff --git a/doc/README.iscsi b/doc/README.iscsi
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+# iSCSI booting with U-Boot and iPXE
+
+## Motivation
+
+U-Boot has only a reduced set of supported network protocols. The focus for
+network booting has been on UDP based protocols. A TCP stack and HTTP support
+are expected to be integrated in 2018 together with a wget command.
+
+For booting a diskless computer this leaves us with BOOTP or DHCP to get the
+address of a boot script. TFTP or NFS can be used to load the boot script, the
+operating system kernel and the initial file system (initrd).
+
+These protocols are insecure. The client cannot validate the authenticity
+of the contacted servers. And the server cannot verify the identity of the
+client.
+
+Furthermore the services providing the operating system loader or kernel are
+not the ones that the operating system typically will use. Especially in a SAN
+environment this makes updating the operating system a hassle. After installing
+a new kernel version the boot files have to be copied to the TFTP server
+directory.
+
+The HTTPS protocol provides certificate based validation of servers. Sensitive
+data like passwords can be securely transmitted.
+
+The iSCSI protocol is used for connecting storage attached networks. It
+provides mutual authentication using the CHAP protocol. It typically runs on
+a TCP transport.
+
+Thus a better solution than DHCP/TFTP/NFS boot would be to load a boot script
+via HTTPS and to download any other files needed for booting via iSCSI from the
+same target where the operating system is installed.
+
+An alternative to implementing these protocols in U-Boot is to use an existing
+software that can run on top of U-Boot. iPXE is the "swiss army knife" of
+network booting. It supports both HTTPS and iSCSI. It has a scripting engine for
+fine grained control of the boot process and can provide a command shell.
+
+iPXE can be built as an EFI application (named snp.efi) which can be loaded and
+run by U-Boot.
+
+## Boot sequence
+
+U-Boot loads the EFI application iPXE snp.efi using the bootefi command. This
+application has network access via the simple network protocol offered by
+U-Boot.
+
+iPXE executes its internal script. This script may optionally chain load a
+secondary boot script via HTTPS or open a shell.
+
+For the further boot process iPXE connects to the iSCSI server. This includes
+the mutual authentication using the CHAP protocol. After the authentication iPXE
+has access to the iSCSI targets.
+
+For a selected iSCSI target iPXE sets up a handle with the block IO protocol. It
+uses the ConnectController boot service of U-Boot to request U-Boot to connect a
+file system driver. U-Boot reads from the iSCSI drive via the block IO protocol
+offered by iPXE. It creates the partition handles and installs the simple file
+protocol. Now iPXE can call the simple file protocol to load Grub. U-Boot uses
+the block IO protocol offered by iPXE to fulfill the request.
+
+Once Grub is started it uses the same block IO protocol to load Linux. Via
+the EFI stub Linux is called as an EFI application.
+
+```
+               +--------+          +--------+
+               |        | Runs     |        |
+               | U-Boot |=========>| iPXE   |
+               | EFI    |          | snp.efi|
++--------+     |        | DHCP     |        |
+|        |<====|********|<=========|        |
+| DHCP   |     |        | Get IP   |        |
+| Server |     |        | Address  |        |
+|        |====>|********|=========>|        |
++--------+     |        | Response |        |
+               |        |          |        |
+               |        |          |        |
++--------+     |        | HTTPS    |        |
+|        |<====|********|<=========|        |
+| HTTPS  |     |        | Load     |        |
+| Server |     |        | Script   |        |
+|        |====>|********|=========>|        |
++--------+     |        |          |        |
+               |        |          |        |
+               |        |          |        |
++--------+     |        | iSCSI    |        |
+|        |<====|********|<=========|        |
+| iSCSI  |     |        | Auth     |        |
+| Server |====>|********|=========>|        |
+|        |     |        |          |        |
+|        |     |        | Loads    |        |
+|        |<====|********|<=========|        |        +--------+
+|        |     |        | Grub     |        | Runs   |        |
+|        |====>|********|=========>|        |=======>| Grub   |
+|        |     |        |          |        |        |        |
+|        |     |        |          |        |        |        |
+|        |     |        |          |        | Loads  |        |
+|        |<====|********|<=========|********|<=======|        |      +--------+
+|        |     |        |          |        | Linux  |        | Runs |        |
+|        |====>|********|=========>|********|=======>|        |=====>| Linux  |
+|        |     |        |          |        |        |        |      |        |
++--------+     +--------+          +--------+        +--------+      |        |
+                                                                     |        |
+                                                                     |        |
+                                                                     | ~ ~ ~ ~|
+```
+
+## Security
+
+The iSCSI protocol is not encrypted. The traffic could be secured using IPsec
+but neither U-Boot nor iPXE does support this. So we should at least separate
+the iSCSI traffic from all other network traffic. This can be achieved using a
+virtual local area network (VLAN).
+
+## Configuration
+
+### iPXE
+
+For running iPXE on arm64 the bin-arm64-efi/snp.efi build target is needed.
+
+    git clone http://git.ipxe.org/ipxe.git
+    cd ipxe/src
+    make bin-arm64-efi/snp.efi -j6 EMBED=myscript.ipxe
+
+The available commands for the boot script are documented at:
+
+http://ipxe.org/cmd
+
+Credentials are managed as environment variables. These are described here:
+
+http://ipxe.org/cfg
+
+iPXE by default will put the CPU to rest when waiting for input. U-Boot does
+not wake it up due to missing interrupt support. To avoid this behavior create
+file src/config/local/nap.h.
+
+    /* nap.h */
+    #undef NAP_EFIX86
+    #undef NAP_EFIARM
+    #define NAP_NULL
+
+The supported commands in iPXE are controlled by an include, too. Putting the
+following into src/config/local/general.h is sufficient for most use cases.
+
+    /* general.h */
+    #define NSLOOKUP_CMD            /* Name resolution command */
+    #define PING_CMD                /* Ping command */
+    #define NTP_CMD                 /* NTP commands */
+    #define VLAN_CMD                /* VLAN commands */
+    #define IMAGE_EFI               /* EFI image support */
+    #define DOWNLOAD_PROTO_HTTPS    /* Secure Hypertext Transfer Protocol */
+    #define DOWNLOAD_PROTO_FTP      /* File Transfer Protocol */
+    #define DOWNLOAD_PROTO_NFS      /* Network File System Protocol */
+    #define DOWNLOAD_PROTO_FILE     /* Local file system access */
+
+## Links
+
+* https://ipxe.org - iPXE open source boot firmware
+* https://www.gnu.org/software/grub/ - GNU Grub (Grand Unified Bootloader)