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

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Date:   Wed, 22 Mar 2023 11:45:18 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Peter Zijlstra <peterz@...radead.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 Wed, Mar 22, 2023 at 04:15:32PM +0100, Peter Zijlstra wrote:
> 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.

So this patch is more of an RFC than the others.  I'm not fully
convinced myself, but I very much liked the removed lines and simpler
interface.

> > 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.

I disagree, to me NULL means "nop the function (including any return
value)".  Nice and easy.

Keeping those other ret0 defines around would negate some of the cool
deletions.

-- 
Josh