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

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Message-ID: <ZS0GQoEsG/20IvnE@gmail.com>
Date:   Mon, 16 Oct 2023 11:45:38 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Breno Leitao <leitao@...ian.org>, tglx@...utronix.de, bp@...en8.de,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, leit@...a.com,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] x86/bugs: Add a separate config for each mitigation


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

> On Fri, Oct 13, 2023 at 12:50:59PM +0200, Ingo Molnar wrote:
> > 
> > * Josh Poimboeuf <jpoimboe@...nel.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