Re: [PATCH v4] x86/bugs: Add a separate config for each mitigation

[Ingo Molnar <mingo@kernel.org>]

* Josh Poimboeuf <jpoimboe@kernel.org> wrote:

> On Fri, Oct 13, 2023 at 12:50:59PM +0200, Ingo Molnar wrote:
> > 
> > * Josh Poimboeuf <jpoimboe@kernel.org> wrote:
> > 
> > > On Thu, Oct 12, 2023 at 08:51:24PM +0200, Ingo Molnar wrote:
> > > > > Another way to avoid ifdeffery:
> > > > > 
> > > > > static enum retbleed_mitigation_cmd retbleed_cmd __ro_after_init =
> > > > > 	IS_ENABLED(CONFIG_MITIGATION_RETBLEED) ? RETBLEED_CMD_AUTO : RETBLEED_CMD_OFF;
> > > > 
> > > > I think we could make it a simple:
> > > > 
> > > > 	static enum retbleed_mitigation_cmd retbleed_cmd __ro_after_init = IS_ENABLED(CONFIG_MITIGATION_RETBLEED);
> > > > 
> > > > Because RETBLEED_CMD_AUTO && RETBLEED_CMD_OFF maps naturally to 1 and 0. 
> > > > Maybe add a comment to the enum to maintain this property in the future 
> > > > too.
> > > 
> > > Hm, that both obfuscates the default and makes it fragile.  The fact
> > > that it would need a comment to try to prevent breaking it in the future
> > > is a clue that maybe we shouldn't do it ;-)
> > 
> > Can be enforced with BUILD_BUG_ON().
> 
> That replaces fragility with brittleness.  If we change a default then
> we have to go rearrange the corresponding enum, and update the
> BUILD_BUG_ONs.

How realistic is that? A world in which an enum named '*_OFF' isn't zero 
and the most obvious second enum isn't 'auto' would be unconditionally sad 
IMO...

> More importantly, it's still less readable because the reader now has to 
> go read the enum values to cross-reference the hard-coded values of 0 and 
> 1 with the enums which are used everywhere else.

They'd have to do that anyway, to make sense of the enum jungle.

Thanks,

	Ingo