Re: [PATCH v2] x86/cpu: replacing the open-coded shift with BIT(x)

[Jason A. Donenfeld]

On Tue, Nov 01, 2022 at 07:14:18PM +0800, Gaosheng Cui wrote:
> Replace the open-coded shift with BIT(x) to make the code a bit
> more self-documenting, at the same time, fix some useless warnings.

Others might feel differently and that's fine, but I always found the
BIT() thing so much less clear than doing 1<<n, which is not only a
pattern that I recognize as builtin to my brain, but also provides a
direct description of what's happening, "shift a 1 over n times",
leaving no off-by-one ambiguity about it. If anything I'd like to see
the BIT() macro expanded throughout and then removed entirely.

Probably just me though. You can safely ignore my opinion :).

Jason


> 
> Signed-off-by: Gaosheng Cui <cuigaosheng1@xxxxxxxxxx>
> ---
> v2:
> - Change the commit msg, remove the UBSAN warning calltrace, and
>   merge patch "x86/cpu: fix undefined behavior in bit shift for
>   intel_detect_tlb" with it. Thanks!
>  arch/x86/kernel/cpu/amd.c     | 2 +-
>  arch/x86/kernel/cpu/centaur.c | 2 +-
>  arch/x86/kernel/cpu/hygon.c   | 2 +-
>  arch/x86/kernel/cpu/intel.c   | 4 ++--
>  arch/x86/kernel/cpu/proc.c    | 2 +-
>  arch/x86/kernel/cpu/zhaoxin.c | 2 +-
>  6 files changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index 860b60273df3..75d82cad323a 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -613,7 +613,7 @@ static void early_init_amd(struct cpuinfo_x86 *c)
>  	 * c->x86_power is 8000_0007 edx. Bit 8 is TSC runs at constant rate
>  	 * with P/T states and does not stop in deep C-states
>  	 */
> -	if (c->x86_power & (1 << 8)) {
> +	if (c->x86_power & BIT(8)) {
>  		set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
>  		set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
>  	}
> diff --git a/arch/x86/kernel/cpu/centaur.c b/arch/x86/kernel/cpu/centaur.c
> index 345f7d905db6..9910bb1d90fd 100644
> --- a/arch/x86/kernel/cpu/centaur.c
> +++ b/arch/x86/kernel/cpu/centaur.c
> @@ -105,7 +105,7 @@ static void early_init_centaur(struct cpuinfo_x86 *c)
>  #ifdef CONFIG_X86_64
>  	set_cpu_cap(c, X86_FEATURE_SYSENTER32);
>  #endif
> -	if (c->x86_power & (1 << 8)) {
> +	if (c->x86_power & BIT(8)) {
>  		set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
>  		set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
>  	}
> diff --git a/arch/x86/kernel/cpu/hygon.c b/arch/x86/kernel/cpu/hygon.c
> index 21fd425088fe..dc473bfbf1b5 100644
> --- a/arch/x86/kernel/cpu/hygon.c
> x86/cpu: fix undefined behavior in bit shift for intel_detect_tlb+++ b/arch/x86/kernel/cpu/hygon.c
> @@ -251,7 +251,7 @@ static void early_init_hygon(struct cpuinfo_x86 *c)
>  	 * c->x86_power is 8000_0007 edx. Bit 8 is TSC runs at constant rate
>  	 * with P/T states and does not stop in deep C-states
>  	 */
> -	if (c->x86_power & (1 << 8)) {
> +	if (c->x86_power & BIT(8)) {
>  		set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
>  		set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
>  	}
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index 2d7ea5480ec3..2bdf6d601a6f 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -286,7 +286,7 @@ static void early_init_intel(struct cpuinfo_x86 *c)
>  	 * It is also reliable across cores and sockets. (but not across
>  	 * cabinets - we turn it off in that case explicitly.)
>  	 */
> -	if (c->x86_power & (1 << 8)) {
> +	if (c->x86_power & BIT(8)) {
>  		set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
>  		set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
>  	}
> @@ -945,7 +945,7 @@ static void intel_detect_tlb(struct cpuinfo_x86 *c)
>  
>  		/* If bit 31 is set, this is an unknown format */
>  		for (j = 0 ; j < 3 ; j++)
> -			if (regs[j] & (1 << 31))
> +			if (regs[j] & BIT(31))
>  				regs[j] = 0;
>  
>  		/* Byte 0 is level count, not a descriptor */
> diff --git a/arch/x86/kernel/cpu/proc.c b/arch/x86/kernel/cpu/proc.c
> index 099b6f0d96bd..efa1d39c4f25 100644
> --- a/arch/x86/kernel/cpu/proc.c
> +++ b/arch/x86/kernel/cpu/proc.c
> @@ -135,7 +135,7 @@ static int show_cpuinfo(struct seq_file *m, void *v)
>  
>  	seq_puts(m, "power management:");
>  	for (i = 0; i < 32; i++) {
> -		if (c->x86_power & (1 << i)) {
> +		if (c->x86_power & BIT(i)) {
>  			if (i < ARRAY_SIZE(x86_power_flags) &&
>  			    x86_power_flags[i])
>  				seq_printf(m, "%s%s",
> diff --git a/arch/x86/kernel/cpu/zhaoxin.c b/arch/x86/kernel/cpu/zhaoxin.c
> index 05fa4ef63490..34a8a460f8f4 100644
> --- a/arch/x86/kernel/cpu/zhaoxin.c
> +++ b/arch/x86/kernel/cpu/zhaoxin.c
> @@ -61,7 +61,7 @@ static void early_init_zhaoxin(struct cpuinfo_x86 *c)
>  #ifdef CONFIG_X86_64
>  	set_cpu_cap(c, X86_FEATURE_SYSENTER32);
>  #endif
> -	if (c->x86_power & (1 << 8)) {
> +	if (c->x86_power & BIT(8)) {
>  		set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
>  		set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
>  	}
> -- 
> 2.25.1
>