Hi,
Unused space gets returned to the e820 map before boot, so there is generally not much harm in increasing it.
Ah, missed that detail ...
So simply bumping it to 1M or so is fine and does not waste memory?
What's using the ZoneHigh region that is so large that we need to expand it?
zonehigh is 256k. Largest allocation usually are the acpi tables (128k for a typical q35 config) and that is the one which typically fails when the other allocations sum up to > 128k so there is less than 128k free space left.
Lots of vcpus (~800 IIRC) leading to large smbios tables are one way to trigger this.
I've also seen allocation failures with many disk devices, although I'm not sure whenever that was zonehigh or zonelow. And the change to only initialize bootable disks should help with that one too.
take care, Gerd