linux-kernel - Re: [PATCH v3 14/19] x86/coco: Make cc_set

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Message-ID: <CAMj1kXECr7SDTynefURwGYVYnAY+i+TDjW-i9hqf_5EFV2Qeqw@mail.gmail.com>
Date: Wed, 31 Jan 2024 00:36:15 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Kevin Loughlin <kevinloughlin@...gle.com>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, 
	Tom Lendacky <thomas.lendacky@....com>, Dionna Glaze <dionnaglaze@...gle.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>, 
	Arnd Bergmann <arnd@...db.de>, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
	Kees Cook <keescook@...omium.org>, Brian Gerst <brgerst@...il.com>, linux-arch@...r.kernel.org, 
	llvm@...ts.linux.dev
Subject: Re: [PATCH v3 14/19] x86/coco: Make cc_set_mask() static inline

On Wed, 31 Jan 2024 at 00:16, Kevin Loughlin <kevinloughlin@...gle.com> wrote:
>
> On Mon, Jan 29, 2024 at 10:06 AM Ard Biesheuvel <ardb+git@...gle.com> wrote:
> >
> > From: Ard Biesheuvel <ardb@...nel.org>
> >
> > Setting the cc_mask global variable may be done early in the boot while
> > running fromm a 1:1 translation. This code is built with -fPIC in order
> > to support this.
> >
> > Make cc_set_mask() static inline so it can execute safely in this
> > context as well.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > ---
> >  arch/x86/coco/core.c        | 7 +------
> >  arch/x86/include/asm/coco.h | 8 +++++++-
> >  2 files changed, 8 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/x86/coco/core.c b/arch/x86/coco/core.c
> > index eeec9986570e..d07be9d05cd0 100644
> > --- a/arch/x86/coco/core.c
> > +++ b/arch/x86/coco/core.c
> > @@ -14,7 +14,7 @@
> >  #include <asm/processor.h>
> >
> >  enum cc_vendor cc_vendor __ro_after_init = CC_VENDOR_NONE;
> > -static u64 cc_mask __ro_after_init;
> > +u64 cc_mask __ro_after_init;
> >
> >  static bool noinstr intel_cc_platform_has(enum cc_attr attr)
> >  {
> > @@ -148,8 +148,3 @@ u64 cc_mkdec(u64 val)
> >         }
> >  }
> >  EXPORT_SYMBOL_GPL(cc_mkdec);
> > -
> > -__init void cc_set_mask(u64 mask)
> > -{
> > -       cc_mask = mask;
> > -}
> > diff --git a/arch/x86/include/asm/coco.h b/arch/x86/include/asm/coco.h
> > index 6ae2d16a7613..ecc29d6136ad 100644
> > --- a/arch/x86/include/asm/coco.h
> > +++ b/arch/x86/include/asm/coco.h
> > @@ -13,7 +13,13 @@ enum cc_vendor {
> >  extern enum cc_vendor cc_vendor;
> >
> >  #ifdef CONFIG_ARCH_HAS_CC_PLATFORM
> > -void cc_set_mask(u64 mask);
> > +static inline void cc_set_mask(u64 mask)
>
> In the inline functions I changed/added to core.c in [0], I saw an
> objtool warning on clang builds when using inline instead of
> __always_inline; I did not see the same warning for gcc . Should we
> similarly use __always_inline to strictly-enforce here?
>
> [0] https://lore.kernel.org/lkml/20240130220845.1978329-2-kevinloughlin@google.com/#Z31arch:x86:coco:core.c

This assembles to a single instruction

movq %rsi, cc_mask(%rip)

and the definition is in a header file, so I'm not convinced it makes
a different.

And looking at your series, I think there is no need to modify coco.c
at all if you just take this patch instead: the other code in that
file should not be called early at all (unless our downstream has
substantial changes there)