Booting from real channel-attached devices on s390x

s390 hardware IPL

The s390 hardware IPL process consists of the following steps.

  1. A READ IPL ccw is constructed in memory location 0x0. This ccw, by definition, reads the IPL1 record which is located on the disk at cylinder 0 track 0 record 1. Note that the chain flag is on in this ccw so when it is complete another ccw will be fetched and executed from memory location 0x08.

  2. Execute the Read IPL ccw at 0x00, thereby reading IPL1 data into 0x00. IPL1 data is 24 bytes in length and consists of the following pieces of information: [psw][read ccw][tic ccw]. When the machine executes the Read IPL ccw it read the 24-bytes of IPL1 to be read into memory starting at location 0x0. Then the ccw program at 0x08 which consists of a read ccw and a tic ccw is automatically executed because of the chain flag from the original READ IPL ccw. The read ccw will read the IPL2 data into memory and the TIC (Transfer In Channel) will transfer control to the channel program contained in the IPL2 data. The TIC channel command is the equivalent of a branch/jump/goto instruction for channel programs.

    NOTE: The ccws in IPL1 are defined by the architecture to be format 0.

  3. Execute IPL2. The TIC ccw instruction at the end of the IPL1 channel program will begin the execution of the IPL2 channel program. IPL2 is stage-2 of the boot process and will contain a larger channel program than IPL1. The point of IPL2 is to find and load either the operating system or a small program that loads the operating system from disk. At the end of this step all or some of the real operating system is loaded into memory and we are ready to hand control over to the guest operating system. At this point the guest operating system is entirely responsible for loading any more data it might need to function.

    NOTE: The IPL2 channel program might read data into memory location 0x0 thereby overwriting the IPL1 psw and channel program. This is ok as long as the data placed in location 0x0 contains a psw whose instruction address points to the guest operating system code to execute at the end of the IPL/boot process.

    NOTE: The ccws in IPL2 are defined by the architecture to be format 0.

  4. Start executing the guest operating system. The psw that was loaded into memory location 0x0 as part of the ipl process should contain the needed flags for the operating system we have loaded. The psw’s instruction address will point to the location in memory where we want to start executing the operating system. This psw is loaded (via LPSW instruction) causing control to be passed to the operating system code.

In a non-virtualized environment this process, handled entirely by the hardware, is kicked off by the user initiating a “Load” procedure from the hardware management console. This “Load” procedure crafts a special “Read IPL” ccw in memory location 0x0 that reads IPL1. It then executes this ccw thereby kicking off the reading of IPL1 data. Since the channel program from IPL1 will be written immediately after the special “Read IPL” ccw, the IPL1 channel program will be executed immediately (the special read ccw has the chaining bit turned on). The TIC at the end of the IPL1 channel program will cause the IPL2 channel program to be executed automatically. After this sequence completes the “Load” procedure then loads the psw from 0x0.

How this all pertains to QEMU (and the kernel)

In theory we should merely have to do the following to IPL/boot a guest operating system from a DASD device:

  1. Place a “Read IPL” ccw into memory location 0x0 with chaining bit on.

  2. Execute channel program at 0x0.

  3. LPSW 0x0.

However, our emulation of the machine’s channel program logic within the kernel is missing one key feature that is required for this process to work: non-prefetch of ccw data.

When we start a channel program we pass the channel subsystem parameters via an ORB (Operation Request Block). One of those parameters is a prefetch bit. If the bit is on then the vfio-ccw kernel driver is allowed to read the entire channel program from guest memory before it starts executing it. This means that any channel commands that read additional channel commands will not work as expected because the newly read commands will only exist in guest memory and NOT within the kernel’s channel subsystem memory. The kernel vfio-ccw driver currently requires this bit to be on for all channel programs. This is a problem because the IPL process consists of transferring control from the “Read IPL” ccw immediately to the IPL1 channel program that was read by “Read IPL”.

Not being able to turn off prefetch will also prevent the TIC at the end of the IPL1 channel program from transferring control to the IPL2 channel program.

Lastly, in some cases (the zipl bootloader for example) the IPL2 program also transfers control to another channel program segment immediately after reading it from the disk. So we need to be able to handle this case.

What QEMU does

Since we are forced to live with prefetch we cannot use the very simple IPL procedure we defined in the preceding section. So we compensate by doing the following.

  1. Place “Read IPL” ccw into memory location 0x0, but turn off chaining bit.

  2. Execute “Read IPL” at 0x0.

    So now IPL1’s psw is at 0x0 and IPL1’s channel program is at 0x08.

  3. Write a custom channel program that will seek to the IPL2 record and then execute the READ and TIC ccws from IPL1. Normally the seek is not required because after reading the IPL1 record the disk is automatically positioned to read the very next record which will be IPL2. But since we are not reading both IPL1 and IPL2 as part of the same channel program we must manually set the position.

  4. Grab the target address of the TIC instruction from the IPL1 channel program. This address is where the IPL2 channel program starts.

    Now IPL2 is loaded into memory somewhere, and we know the address.

  5. Execute the IPL2 channel program at the address obtained in step #4.

    Because this channel program can be dynamic, we must use a special algorithm that detects a READ immediately followed by a TIC and breaks the ccw chain by turning off the chain bit in the READ ccw. When control is returned from the kernel/hardware to the QEMU bios code we immediately issue another start subchannel to execute the remaining TIC instruction. This causes the entire channel program (starting from the TIC) and all needed data to be refetched thereby stepping around the limitation that would otherwise prevent this channel program from executing properly.

    Now the operating system code is loaded somewhere in guest memory and the psw in memory location 0x0 will point to entry code for the guest operating system.

  6. LPSW 0x0

    LPSW transfers control to the guest operating system and we’re done.