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

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Date:   Fri, 14 Feb 2020 12:08:44 -0500
From:   Qian Cai <cai@....pw>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     pbonzini@...hat.com, vkuznets@...hat.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm/emulate: fix a -Werror=cast-function-type

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.

> 
> > -		else
> > +		if (ctxt->d & Fastop) {
> > +			fastop_t fop = (void *)ctxt->execute;
> > +			rc = fastop(ctxt, fop);
> > +		} else {
> >  			rc = ctxt->execute(ctxt);
> > +		}
> >  		if (rc != X86EMUL_CONTINUE)
> >  			goto done;
> >  		goto writeback;
> > -- 
> > 1.8.3.1
> >