We could use some help from a Qemu expert with an Qemu/ACPI/Linux related problem, working on Coconut SVSM.

See https://github.com/coconut-svsm/svsm/issues/646

Coconut has code to read ACPI information via fw_cfg, and extract the number of guest CPUs from that. That code has been used in the past, but since igvm became the default launch method for SVSM, it was only used in corner-cases, while the information were obtained in some other way (igmv parameter). Now a commit switches back to the fw_cfg+ACPI method. The information returned by it is correct, but strangely Linux (guest) is spitting out ACPI related errors and crashes in some cases before reaching user-space. We do not have any clue how this can be related other than through Qemu behavior. There is no direct way how SVSM can influence the ACPI related behavior of the Linux guest kernel.

The problem seems to be caused by simply reading the ACPI data.

Reverting the bad commit and simply calling the original fw_cfg acpi function causes problems for Linux.