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As mentioned, technically there's no limit. But in practice there is. It all has to do with the overhead you get when running in virtual environments and especially when CPU instructions need to be emulated. I do work with modern z-environments, in a current z/VM virtualization overhead is about 7%, emulation is effectively about 100% overhead, more or less the same is true for any VM/ESA or VM/XA system.

You have to consider SIE processing. () Because of SIE your guest can get access to the processor directly, but when SIE is not available all CPU load needs to be emulated by the host levels. SIE can run three levels. In the past you were running a mainframe in basic mode. You could run three levels with SIE: first level (or native), second level and third level. SIE was available in the 370 XA architecture, so VM/370 doesn't include SIE, you would need VM/XA or newer for that.

Then we got LPAR mode mainframes. Starting zArchitecture LPAR mode is the only mode available. PR/SM is basically the first level of virtualization. So now you can run two SIE levels in an LPAR, the VM Host (now refered to as native or first level) and a guest under VM (now refered to as second level). Any guest running under the second level guest (a third level guest) cannot benefit from SIE processing. Remember, in LPAR mode this third level guest is actually a fourth level guest. So in practice, when running a VM LPAR with a VM guest and a VM or VSE guest under the second level VM, the host now needs to emulate all CPU instructions.

Some 15 years ago we wanted to run a customer in a second level environment. Indeed we could run their VM?(z/VM -> customer z/VM guest -> z/VSE guests) as second level guest, but their VSE systems couldn't use SIE. As a result the load was emulated and CPU usage doubled and also introduced a lot of other timing related issues. That wasn't an option for that customer, but we did run another customer as second level environment for a couple of years and they accepted the increased CPU load for their environment.

Apart from running on the real hardware, we obviously focus on Hercules. So in this case all CPU needs to be emulated, going from x86 hardware to an emulated S360-S370-S390 CPU. Next we run VM in hercules. As stated, SIE is only found in VM/XA and later so SIE is not available in VM/370 and a guest CPU needs to be emulated in any level of virtualization. But for argument sake, Hercules implements the ESA/390 architecture, including SIE, so if you run a newer VM within this system SIE is available. But only for three levels, and the S390 CPU is emulated to begin with.

But technically, you can run Hercules -> VM -> Linux -> Hercules -> Linux etc (or whatever configuration you like). The only thing is, your guest may be very slowwww as the overhead will double with every new level, especially when SIE is not available in any of the levels (VM/370 or any level beyond third level virtualization).

At least you're not the first to try this. I have seen a virtualization up to 7 levels running various configurations of hercules, VM and Linux.

Regards, Berry.

Op 11-03-2025 om 15:48 schreef Alexander Huemer via groups.io: