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qemu-vm: Fix useBootLoader, remove /boot read-only restriction #92122

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merged 5 commits into from Jul 6, 2020
Merged

qemu-vm: Fix useBootLoader, remove /boot read-only restriction #92122

merged 5 commits into from Jul 6, 2020

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nh2
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@nh2 nh2 commented Jul 3, 2020 •
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TODO
Motivation for this change
  • Do not mount /boot read-only, because it is not necessary and only forbids useful things.
  • Fix device name hardcodes on useBootLoader.

See commit messages for details.

Things done
  • Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
  • Built on platform(s)
    • NixOS
    • macOS
    • other Linux distributions
  • Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
  • Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
  • Tested execution of all binary files (usually in ./result/bin/)
  • Determined the impact on package closure size (by running nix path-info -S before and after)
  • Ensured that relevant documentation is up to date
  • Fits CONTRIBUTING.md.

I've tested it via the grub-test-vm in https://github.com/nh2/nixos-vm-examples/tree/0b29937fcb23424b95aa98121c94db02fd9b920d

@nh2 nh2 requested a review from srhb July 3, 2020 02:40
@nh2 nh2 self-assigned this Jul 3, 2020
@nh2 nh2 mentioned this pull request Jul 3, 2020
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LGTM but I very rarely use this -- someone else might have a more educated opinion 😄

@nh2
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nh2 commented Jul 3, 2020

Also FYI @edolstra that this removes the ro mount of /boot in the VM (the commit that added that is very old, since the beginning of the VM stuff).

CCing also the reviewers I put on #85895, @OmnipotentEntity, @Mic92, @symphorien, @emilazy.

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This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-ready-for-review/3032/212

@chkno chkno mentioned this pull request Jul 3, 2020
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Can you add a test that verifies useBootLoader? This both keeps it from breaking again and also really helps other folks working in here to make changes with confidence that they're not breaking stuff.

nixos/modules/virtualisation/qemu-vm.nix
@@ -528,6 +567,8 @@ in

virtualisation.qemu.drives = mkMerge [
(mkIf cfg.useBootLoader [
# The order of this list determines the device names, see
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I fixed this brittle order-determining-device-names thing as part of #72354. Sorry that I didn't aggressively enough pursue reviewers to get that merged to make this change easier. I've split out the cleanup and refactor parts into the separate PR #92205 that can hopefully be reviewed and merged faster.

I think it'd be great if we could have fewer "/dev/..." strings flying around in here, and just one diskInterface == "scsi" test about device names.

Consider reviewing #92205? (Rebasing this atop #92205 would be an excellent way to review #92205!)

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OK, currently testing the rebase.

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@nh2 nh2 Jul 4, 2020 •
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@chkno With #92205 (comment) fixed, this PR rebased on top of yours passes the VM based test I wrote for #85895.

So this is all looking very good, thanks!

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Just adding a note here that I also need a writable boot partition for testing systemd-boot's boot counters. These counters are state which is contained in the filenames of the boot entries under /boot/loader/entries.

If this change is accepted, we should remove this statement from the nixos-rebuild manpage, however:

nixpkgs/nixos/doc/manual/man-nixos-rebuild.xml

Line 308 in 485a993

partition, which is mounted read-only in the VM.

@nh2 nh2 force-pushed the qemu-vm-fix-useBootLoader branch from 1b496d4 to e599834 Compare July 4, 2020 01:27
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nh2 commented Jul 4, 2020

If this change is accepted, we should remove this statement from the nixos-rebuild manpage, however:

Great point, I've added that to the commit.

@nh2 nh2 force-pushed the qemu-vm-fix-useBootLoader branch 2 times, most recently from 010aa57 to 8d5ba86 Compare July 4, 2020 12:37
nh2 added 2 commits July 4, 2020 14:44
@nh2
qemu-vm.nix: Do not mount /boot read-only.
2fa351b 
There does not seem to be a good reason to do this, and it breaks running
`nixos-rebuild boot --install-bootloader` inside the VM.
@nh2
qemu-vm.nix: Fix device name hardcodes on useBootLoader.
5b16d4c 
boot.loader.grub.device` was hardcoded to `bootDevice`, which is
wrong, because that's the device for `/`, and with `useBootLoader`
the boot loader is not on that device.

This bug probably came into existence because of bad naming;
`virtualisation.bootDevice` has description
"The disk to be used for the root filesystem", which is very confusing;
it should be `.rootDevice` then!
Unfortunately, the description is right and the attribute name is wrong,
so it is not easy to change this without deprecation.

This commit ensures that even if you use `useBootLoader` and
`diskInterface == "scsi"`, the created VM can boot through, and can run
`nixos-rebuild afterwards.

It also adds extra commentary to explain what's going on in this module
in general in relation to `useBootLoader`.
@nh2 nh2 force-pushed the qemu-vm-fix-useBootLoader branch from 8d5ba86 to 5b16d4c Compare July 4, 2020 12:48
@nh2 nh2 requested a review from chkno July 4, 2020 13:08
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nh2 commented Jul 4, 2020

I've addressed all feedback and re-tested, will probably merge tomorrow.

@nh2 nh2 merged commit d4d9d9c into NixOS:master Jul 6, 2020
@chkno chkno mentioned this pull request Jul 6, 2020
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0.kind: bug 0.kind: enhancement 6.topic: nixos 8.has: clean-up 8.has: documentation 8.has: module (update) 10.rebuild-darwin: 0 10.rebuild-linux: 1-10 10.rebuild-linux: 1
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