linux-kernel - Re: [PATCH] x86/tdx: Fix __noreturn build warning around __tdx_hypercall

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Date:   Mon, 18 Sep 2023 03:57:01 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "mingo@...nel.org" <mingo@...nel.org>
CC:     "Hansen, Dave" <dave.hansen@...el.com>,
        "bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jpoimboe@...nel.org" <jpoimboe@...nel.org>,
        "nik.borisov@...e.com" <nik.borisov@...e.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Subject: Re: [PATCH] x86/tdx: Fix __noreturn build warning around
 __tdx_hypercall_failed()

On Fri, 2023-09-15 at 14:26 +0200, Ingo Molnar wrote:
> * Kai Huang <kai.huang@...el.com> wrote:
> 
> > LKP reported below build warning:
> > 
> >   vmlinux.o: warning: objtool: __tdx_hypercall+0x128: __tdx_hypercall_failed() is missing a __noreturn annotation
> > 
> > Turns out the __noreturn must be annotated to the function declaration
> > but not the function body.
> > 
> > Quoted from PeterZ:
> > 
> > ---
> > FWIW, the reason being that...
> > 
> > The point of noreturn is that the caller should know to stop generating
> > code. For that the declaration needs the attribute, because call sites
> > typically do not have access to the function definition in C.
> > ---
> > 
> > Fix by moving __noreturn annotation from the __tdx_hypercall_failed()
> > body to its declaration, which is in <asm/shared/tdx.h>.
> > 
> > Note <asm/shared/tdx.h> is also included by TDX related assembly files.
> > Include <linux/compiler_attributes.h> only in case of !__ASSEMBLY__
> > otherwise compiling assembly file would trigger build error.
> > 
> > Also, following the objtool documentation, add __tdx_hypercall_failed()
> > to "tools/objtool/noreturns.h".
> > 
> > Fixes: c641cfb5c157 ("x86/tdx: Make TDX_HYPERCALL asm similar to TDX_MODULE_CALL")
> > Reported-by: kernel test robot <lkp@...el.com>
> > Closes: https://lore.kernel.org/oe-kbuild-all/202309140828.9RdmlH2Z-lkp@intel.com/
> > Signed-off-by: Kai Huang <kai.huang@...el.com>
> > ---
> >  arch/x86/coco/tdx/tdx.c           | 2 +-
> >  arch/x86/include/asm/shared/tdx.h | 4 +++-
> >  tools/objtool/noreturns.h         | 1 +
> >  3 files changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> > index 3e6dbd2199cf..4710d8dd700b 100644
> > --- a/arch/x86/coco/tdx/tdx.c
> > +++ b/arch/x86/coco/tdx/tdx.c
> > @@ -38,7 +38,7 @@
> >  #define TDREPORT_SUBTYPE_0	0
> >  
> >  /* Called from __tdx_hypercall() for unrecoverable failure */
> > -noinstr void __noreturn __tdx_hypercall_failed(void)
> > +noinstr void __tdx_hypercall_failed(void)
> >  {
> 
> It's not a bad idea to document the __noreturn nature at the definition 
> site either, so I don't think we should remove it.
> 
> 

Thanks for feedback! I'll fix and send out v2.

Since it's a simple fix, I'll send out v2 shortly.