nixos/lib/qemu-common.nix: add i686-linux and support x86_64 outside x86_64

[RossComputerGuy]

Description of changes

Necessary because qemu-kvm appears to only supports the host machine. This does not help when an aarch64 machine wants to emulate an x86_64 machine but ends up trying qemu-kvm which runs an QEMU ARM virt machine.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[alyssais]

Partial duplicate of #327349.

[alyssais]

Why the change to q35?

[RossComputerGuy]

Consistency with otherHostsGuests.x86_64-darwin.x86_64-linux.

nixpkgs/nixos/lib/qemu-common.nix

Lines 42 to 50 in a7931ad

	otherHostGuestMatrix = {
	aarch64-darwin = {
	aarch64-linux = "${qemuPkg}/bin/qemu-system-aarch64 -machine virt,gic-version=2,accel=hvf:tcg -cpu max";
	inherit (otherHostGuestMatrix.x86_64-darwin) x86_64-linux;
	};
	x86_64-darwin = {
	x86_64-linux = "${qemuPkg}/bin/qemu-system-x86_64 -machine type=q35,accel=hvf:tcg -cpu max";
	};
	};

[alyssais]

But not consistency with the way it worked before, for the common case of not cross compiling? Do we have any knowledge of why that one platform uses q35?

[RossComputerGuy]

Do we have any knowledge of why that one platform uses q35?

Via the blame, I found e0fd131. Which #180222 added that in. From @YorikSar:

Mainly Internet research, trial and error. The q35 was suggested by @willcohen in #108984 (comment). Do you think we should document everything that happens in these parameters?

It seems from this, there might've been a case where type=virt caused issue with people on macOS on Apple's M1 chip and changing it to type=q35 fixed that.