linux-kernel - Re: [PATCH] cpu: microcode: replace 0 with NULL

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Date:   Tue, 26 Nov 2019 14:53:30 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Jules Irenge <jbi.octave@...il.com>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org, x86@...nel.org,
        hpa@...or.com, mingo@...hat.com
Subject: Re: [PATCH] cpu: microcode: replace 0 with NULL

On Tue, Nov 26, 2019 at 12:27:34AM +0000, Jules Irenge wrote:
> Replace 0 with NULL to fix sparse tool  warning
>  warning: Using plain integer as NULL pointer
> 
> Signed-off-by: Jules Irenge <jbi.octave@...il.com>
> ---
>  arch/x86/kernel/cpu/microcode/amd.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/microcode/amd.c b/arch/x86/kernel/cpu/microcode/amd.c
> index a0e52bd00ecc..4934aa7c94e7 100644
> --- a/arch/x86/kernel/cpu/microcode/amd.c
> +++ b/arch/x86/kernel/cpu/microcode/amd.c
> @@ -418,7 +418,7 @@ static int __apply_microcode_amd(struct microcode_amd *mc)
>  static bool
>  apply_microcode_early_amd(u32 cpuid_1_eax, void *ucode, size_t size, bool save_patch)
>  {
> -	struct cont_desc desc = { 0 };
> +	struct cont_desc desc = { NULL };

So my gcc guy says that 0 and NULL are equivalent as designated
initializers in this case. And if you look at the resulting asm, it
doesn't change:

# arch/x86/kernel/cpu/microcode/amd.c:421: 	struct cont_desc desc = { 0 };
	movq	$0, 8(%rsp)	#, desc
	movq	$0, (%rsp)	#, desc
	movq	$0, 16(%rsp)	#, desc
	movq	$0, 24(%rsp)	#, desc

But what I'd prefer actually is, if you do them like this:

			... = { 0,  };

because:

1. It is clear that the memory for the struct is being cleared
2. The following ones - the ones after "," are missing too, on purpose,
   because they're being cleared too.

Also pls add that explanation to the commit message.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette