Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.37 as permitted sender) client-ip=23.128.96.37;
Date:   Sun, 15 Oct 2023 07:11:47 -0700
From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     Ingo Molnar <mingo@kernel.org>
Cc:     Breno Leitao <leitao@debian.org>, tglx@linutronix.de, bp@alien8.de,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>, leit@meta.com,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4] x86/bugs: Add a separate config for each mitigation
Message-ID: <20231015141147.qeczgcfnl73zcqao@treble>
References: <20231010103028.4192223-1-leitao@debian.org>
 <ZSca08rnmZfkONEH@gmail.com>
 <ZSfucR6docdnLznb@gmail.com>
 <20231012170548.o5vi4kgpvpjyld7s@treble>
 <ZShALJDaxJ9VJvek@gmail.com>
 <20231012204347.4aei5jr64fsv2iv5@treble>
 <ZSkhE2GWfGRW+9Hh@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <ZSkhE2GWfGRW+9Hh@gmail.com>
Precedence: bulk

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.

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.

-- 
Josh