Re: [PATCH 2/2] x86/paravirt: add extra clobbers with ZERO_CALL_USED_REGS enabled

[Nathan Chancellor <nathan@kernel.org>]

On Wed, Sep 07, 2022 at 10:50:03AM +0200, Peter Zijlstra wrote:
> On Tue, Sep 06, 2022 at 11:00:07PM -0700, Nick Desaulniers wrote:
> > On Sun, Sep 4, 2022 at 11:02 PM Bill Wendling <morbo@google.com> wrote:
> > >
> > > On Sat, Sep 3, 2022 at 12:18 AM Kees Cook <keescook@chromium.org> wrote:
> > > >
> > > > On Fri, Sep 02, 2022 at 09:37:50PM +0000, Bill Wendling wrote:
> > > > > [...]
> > > > >         callq   *pv_ops+536(%rip)
> > > >
> > > > Do you know which pv_ops function is this? I can't figure out where
> > > > pte_offset_kernel() gets converted into a pv_ops call....
> > > >
> > > This one is _paravirt_ident_64, I believe. I think that the original
> > > issue Nathan was seeing was with another seemingly innocuous function.
> > 
> > _paravirt_ident_64 is marked noinstr, which makes me suspect that it
> > really needs to not be touched at all by the compiler for
> > these...special features.
> 
> My source tree sayeth:
> 
>   u64 notrace _paravirt_ident_64(u64 x)
> 
> And that function is only ever called at boot, after alternatives runs
> it's patched with:
> 
>   mov %_ASM_ARG1, %_ASM_AX
> 
> Anyway, if you want to take it away from the compiler, something like
> so should do.

This appears to work fine for me in QEMU, as I can still boot with
CONFIG_ZERO_CALL_USED_REGS and spawn a nested guest without any issues.

> diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
> index 7ca2d46c08cc..8922e2887779 100644
> --- a/arch/x86/kernel/paravirt.c
> +++ b/arch/x86/kernel/paravirt.c
> @@ -80,11 +80,16 @@ static unsigned paravirt_patch_call(void *insn_buff, const void *target,
>  }
>  
>  #ifdef CONFIG_PARAVIRT_XXL
> -/* identity function, which can be inlined */
> -u64 notrace _paravirt_ident_64(u64 x)
> -{
> -	return x;
> -}
> +extern u64 _paravirt_ident_64(u64 x);
> +asm (".pushsection .entry.text, \"ax\"\n"
> +     ".global _paravirt_ident_64\n"
> +     "_paravirt_ident_64:\n\t"
> +     ASM_ENDBR
> +     "mov %" _ASM_ARG1 ", %" _ASM_AX "\n\t"
> +     ASM_RET
> +     ".size _paravirt_ident_64, . - _paravirt_ident_64\n\t"
> +     ".type _paravirt_ident_64, @function\n\t"
> +     ".popsection");
>  #endif
>  
>  DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);
>