On 01.03.21 18:52, Alan Haff wrote: > Greetings, > > Will QEMU using TCG instead of KVM emulate CPU features that are missing on > the host processor? Specifically, will it emulate ESA/390 architecture on a > z14 or z15 with this config: > > <cpu mode='custom' match='exact' check='none'> > <model fallback='allow'>z900.3</model> > </cpu> > > for example?
I think TCG (like KVM) never implemented ESA/S390 but only zarchitecture. > > I've already confirmed that config doesn't provide features from newer CPU > models but we don't have a z14/z15 so I can't confirm that it will emulate > older features on newer CPUs. > > I know that performance will be worse (maybe much worse) with TCG instead of > KVM but I'm investigating this as a short-term solution for a specific use > case. If you want to test some software if this is runnable on older hardware you can also use the CPU model with KVM. While not perfect in every regard, the hardware provides a so call virtual architecture level that covers 2 additional generations. This was introduced to provide live migration across different generations. So when you for example use <model>zEC12</model> on a z14 then KVM will instruct the hardware to disallow instructions introduced with z13 and z14. On a z15 you can go back to z13 for "perfect" fencing. If you go back to zEC12 on a z15 then KVM will instruct the hardware to fence everything from z14 and z15 and it will tell the guest that only zEC12 facilities via stfle) are there. If code ignores stfle then it might be able to execution some instructions, though. ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO LINUX-390 or visit http://www2.marist.edu/htbin/wlvindex?LINUX-390
