linux-kernel - Re: [PATCH] kvm/emulate: fix a -Werror=cast-function-type

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Message-ID: <CALMp9eTRn-46oKg5a9h79EZOvHGwT=8ZZN15Zmy5NUYsd+r8wQ@mail.gmail.com>
Date:   Fri, 14 Feb 2020 09:40:41 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Qian Cai <cai@....pw>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kvm/emulate: fix a -Werror=cast-function-type

On Fri, Feb 14, 2020 at 9:08 AM Qian Cai <cai@....pw> wrote:
>
> On Fri, 2020-02-14 at 08:59 -0800, Sean Christopherson wrote:
> > On Fri, Feb 14, 2020 at 10:56:08AM -0500, Qian Cai wrote:
> > > arch/x86/kvm/emulate.c: In function 'x86_emulate_insn':
> > > arch/x86/kvm/emulate.c:5686:22: error: cast between incompatible
> > > function types from 'int (*)(struct x86_emulate_ctxt *)' to 'void
> > > (*)(struct fastop *)' [-Werror=cast-function-type]
> > >     rc = fastop(ctxt, (fastop_t)ctxt->execute);
> > >
> > > Fixes: 3009afc6e39e ("KVM: x86: Use a typedef for fastop functions")
> > > Signed-off-by: Qian Cai <cai@....pw>
> > > ---
> > >  arch/x86/kvm/emulate.c | 8 +++++---
> > >  1 file changed, 5 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> > > index ddbc61984227..17ae820cf59d 100644
> > > --- a/arch/x86/kvm/emulate.c
> > > +++ b/arch/x86/kvm/emulate.c
> > > @@ -5682,10 +5682,12 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
> > >             ctxt->eflags &= ~X86_EFLAGS_RF;
> > >
> > >     if (ctxt->execute) {
> > > -           if (ctxt->d & Fastop)
> > > -                   rc = fastop(ctxt, (fastop_t)ctxt->execute);
> >
> > Alternatively, can we do -Wno-cast-function-type?  That's a silly warning
> > IMO.
>
> I am doing W=1 on linux-next where some of the warnings might be silly but the
> recent commit changes all warnings to errors forces me having to silence those
> somehow.
>
> >
> > If not, will either of these work?
> >
> >                       rc = fastop(ctxt, (void *)ctxt->execute);
> >
> > or
> >                       rc = fastop(ctxt, (fastop_t)(void *)ctxt->execute);
>
> I have no strong preference. I originally thought just to go back the previous
> code style where might be more acceptable, but it is up to maintainers.

It seems misguided to define a local variable just to get an implicit
cast from (void *) to (fastop_t). Sean's first suggestion gives you
the same implicit cast without the local variable. The second
suggestion makes both casts explicit.