linux-kernel - Re: [PATCH v2 11/11] static_call: Remove DEFINE_STATIC_CALL

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Message-ID: <20230322151532.GG2357380@hirez.programming.kicks-ass.net>
Date:   Wed, 22 Mar 2023 16:15:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Jason Baron <jbaron@...mai.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Will McVicker <willmcvicker@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 11/11] static_call: Remove DEFINE_STATIC_CALL_RET0()

On Tue, Mar 21, 2023 at 09:00:17PM -0700, Josh Poimboeuf wrote:
> NULL and RET0 static calls are both slightly different ways of nopping a
> static call.  A not-insignificant amount of code and complexity is spent
> maintaining them separately.  It's also somewhat tricky for the user who
> has to try to remember to use the correct one for the given function
> type.

Well, I have very little sympathy for that argument. The return type
should be a big frigging clue.

> Simplify things all around by just combining them, such that NULL static
> calls always return 0.
> 
> While it doesn't necessarily make sense for void-return functions to
> return 0, it's pretty much harmless.  The return value register is
> already callee-clobbered, and an extra "xor %eax, %eax" shouldn't affect
> performance (knock on wood).

Urgh.. OTOH I do like the lines removes.

> This "do nothing return 0" default should work for the vast majority of
> NULL cases.  Otherwise it can be easily overridden with a user-specified
> function which panics or returns 0xdeadbeef or does whatever one wants.
> 
> This simplifies the static call code and also tends to help simplify
> users' code as well.

Can we at least keep the DEFINE_STATIC_CALL_RET0() and
__static_call_return0 as aliases? It reads really daft to use _NULL or
__static_call_nop for non-void functions.