Re: [PATCH v2 1/1] Add support for grub-emu to kexec Linux menu entries

[Robbie Harwood <rharwood@redhat.com>]

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Raymund Will <rw@suse.de> writes:

> Hi,
>
> please let me try to add a bit of context and explain three IMHO
> crucial points of the proposed patch.
>
> First, it is meant to work without any changes to config-files
> on 'linux'-systems, simply by calling `grub-emu --kexec`.
> And, called this way, it actually uses `systemctl kexec` exclusively
> to instruct `systemd` to perform the "kexec"-reboot in a sane and
> safe manner.
>
> Second, it only supports a very limited set of commands in `grub.cfg`,
> as `grub-emu` can not implement the full functionality of a firmware-
> loaded `grub` binary (like raw-device access) and it hinges massively
> on proper `kexec`-support from the machine/firmware, so unfortunately
> it won't be universally useful,
>
> Third, for use in a "pre-boot environment" (i.e. initrd/chroot), which
> has full control over it's state, but no (fully) working `systemd`,
> a call to `grub-emu --kexec --kexec` changes the behavior to allow a
> fall-forward to `kexec -e`.  As a safe-guard for the adventurously
> minded `systemctl kexec` is still tried first!
>
> This third point describes the use-case the original patch-set was
> developed for and the second doesn't hurt (much) on the systems it
> is deployed/used in the field.  But the first was found to be really
> useful, especially on machines, which can reliably `kexec`, but are
> dead slow through cold-boot (think "huge memory", "tons of devices")
> and/or have "exotic" console setups ("3215" anybody?).  Note that,
> as the boot configuration (namely `grub.cfg`) wasn't changed, regular
> boot can't be affected by this short-cut (particularly, when `kexec`
> might have failed).
>
> Granted, the duplication of `--kexec` to signify "force it", might
> as well be spelled out as `--force-kexec` (or something similar).
> (But that change will provoke inconsistencies during an indefinite
> migration phase, where pre-boot images don't match binaries in the
> root filesystem, notably, when rollback snapshots come into play.)

Passing --kexec twice (or --force-kexec) doesn't appear to change
anything in the versions of this patch I can easily find.  We could add
the behavior you're describing though - Daniel, would that help with
your concerns about it?

> Config-overrides in `grub.cfg` in turn would be a nice addition, but
> are relatively expensive to implement, as they'd probably need to be
> parsed and split into an array for `grub_util_exec()`, right?

Yes.  It's inevitably best-effort, especially if we can't depend on a
working shell.

> But, please, still leave sane defaults in the binary, for out-of-
> the-box, no-config-changes-necessary usage, pretty please!
>
> Would it be possible to re-evaluate the proposed patch with this
> in mind?
>
> PS: in light of my statements above, the "description" of this
> patch definitely needs re-wording...

Any interest in doing that, or should I?

Be well,
--Robbie

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