Powerpc instructions must be word-aligned. Currently,
there is an alignment of 16 bytes (by default), and it is
much more than what is required for powerpc (4 bytes).

The default expansion of __ALIGN() macro is:
#define __ALIGN       .align 4,0x90

Since Powerpc Linux does not require a 16 byte alignment,
override __ALIGN() and __ALIGN_STR() macros to use required
4 byte alignment.

Signed-off-by: Sathvika Vasireddy <sv@linux.ibm.com>
---
 arch/powerpc/include/asm/linkage.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/powerpc/include/asm/linkage.h b/arch/powerpc/include/asm/linkage.h
index b71b9582e754..8df88fe61438 100644
--- a/arch/powerpc/include/asm/linkage.h
+++ b/arch/powerpc/include/asm/linkage.h
@@ -2,8 +2,12 @@
 #ifndef _ASM_POWERPC_LINKAGE_H
 #define _ASM_POWERPC_LINKAGE_H
 
+#include <linux/stringify.h>
 #include <asm/types.h>
 
+#define __ALIGN			.align 2
+#define __ALIGN_STR		__stringify(__ALIGN)
+
 #ifdef CONFIG_PPC64_ELF_ABI_V1
 #define cond_syscall(x) \
 	asm ("\t.weak " #x "\n\t.set " #x ", sys_ni_syscall\n"		\
-- 
2.31.1

On Mon, Aug 29, 2022 at 11:22:09AM +0530, Sathvika Vasireddy wrote:
> Powerpc instructions must be word-aligned. Currently,
> there is an alignment of 16 bytes (by default), and it is
> much more than what is required for powerpc (4 bytes).
> 
> The default expansion of __ALIGN() macro is:
> #define __ALIGN       .align 4,0x90
> 
> Since Powerpc Linux does not require a 16 byte alignment,
> override __ALIGN() and __ALIGN_STR() macros to use required
> 4 byte alignment.
> 
> Signed-off-by: Sathvika Vasireddy <sv@linux.ibm.com>
> ---
>  arch/powerpc/include/asm/linkage.h | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/linkage.h b/arch/powerpc/include/asm/linkage.h
> index b71b9582e754..8df88fe61438 100644
> --- a/arch/powerpc/include/asm/linkage.h
> +++ b/arch/powerpc/include/asm/linkage.h
> @@ -2,8 +2,12 @@
>  #ifndef _ASM_POWERPC_LINKAGE_H
>  #define _ASM_POWERPC_LINKAGE_H
>  
> +#include <linux/stringify.h>
>  #include <asm/types.h>
>  
> +#define __ALIGN			.align 2
> +#define __ALIGN_STR		__stringify(__ALIGN)


Related thread:

  https://lkml.kernel.org/r/YxXJswK9QjhCGmPN@hirez.programming.kicks-ass.net

On Mon Aug 29, 2022 at 3:52 PM AEST, Sathvika Vasireddy wrote:
> Powerpc instructions must be word-aligned. Currently,
> there is an alignment of 16 bytes (by default), and it is
> much more than what is required for powerpc (4 bytes).
>
> The default expansion of __ALIGN() macro is:
> #define __ALIGN       .align 4,0x90
>
> Since Powerpc Linux does not require a 16 byte alignment,
> override __ALIGN() and __ALIGN_STR() macros to use required
> 4 byte alignment.

Alignment can be desirable beyond the minimum requirement, for
example 16 byte alignment for functions could be helpful for
instruction fetch. So it should be explained why possible
benefits of the larger alignment are not worth it.

And if you have the patch in a series, it should be explained
why the patch is required for the series if it is not obvious.

Thanks,
Nick

>
> Signed-off-by: Sathvika Vasireddy <sv@linux.ibm.com>
> ---
>  arch/powerpc/include/asm/linkage.h | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/arch/powerpc/include/asm/linkage.h b/arch/powerpc/include/asm/linkage.h
> index b71b9582e754..8df88fe61438 100644
> --- a/arch/powerpc/include/asm/linkage.h
> +++ b/arch/powerpc/include/asm/linkage.h
> @@ -2,8 +2,12 @@
>  #ifndef _ASM_POWERPC_LINKAGE_H
>  #define _ASM_POWERPC_LINKAGE_H
>  
> +#include <linux/stringify.h>
>  #include <asm/types.h>
>  
> +#define __ALIGN			.align 2
> +#define __ALIGN_STR		__stringify(__ALIGN)
> +
>  #ifdef CONFIG_PPC64_ELF_ABI_V1
>  #define cond_syscall(x) \
>  	asm ("\t.weak " #x "\n\t.set " #x ", sys_ni_syscall\n"		\
> -- 
> 2.31.1

"Nicholas Piggin" <npiggin@gmail.com> writes:
> On Mon Aug 29, 2022 at 3:52 PM AEST, Sathvika Vasireddy wrote:
>> Powerpc instructions must be word-aligned. Currently,
>> there is an alignment of 16 bytes (by default), and it is
>> much more than what is required for powerpc (4 bytes).
>>
>> The default expansion of __ALIGN() macro is:
>> #define __ALIGN       .align 4,0x90
>>
>> Since Powerpc Linux does not require a 16 byte alignment,
>> override __ALIGN() and __ALIGN_STR() macros to use required
>> 4 byte alignment.
>
> Alignment can be desirable beyond the minimum requirement, for
> example 16 byte alignment for functions could be helpful for
> instruction fetch. So it should be explained why possible
> benefits of the larger alignment are not worth it.

Using ".align 2" matches what our existing _GLOBAL macro does. So this
change basically just propagates that existing alignment into this new
macro, which is used for similar purposes.

So if we want to increase the alignment we should do that explicitly,
and update _GLOBAL at the same time.

The change log should probably just say ~= "use the same alignment as
the existing _GLOBAL macro".

What's more important, but not mentioned in the change log, is that we
don't want to pad with 0x90, because repeated 0x90s are not a nop or
trap on powerpc.

cheers

Le 29/08/2022 à 07:52, Sathvika Vasireddy a écrit :
> Powerpc instructions must be word-aligned. Currently,
> there is an alignment of 16 bytes (by default), and it is
> much more than what is required for powerpc (4 bytes).
> 
> The default expansion of __ALIGN() macro is:
> #define __ALIGN       .align 4,0x90
> 
> Since Powerpc Linux does not require a 16 byte alignment,
> override __ALIGN() and __ALIGN_STR() macros to use required
> 4 byte alignment.
> 
> Signed-off-by: Sathvika Vasireddy <sv@linux.ibm.com>
> ---
>   arch/powerpc/include/asm/linkage.h | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/linkage.h b/arch/powerpc/include/asm/linkage.h
> index b71b9582e754..8df88fe61438 100644
> --- a/arch/powerpc/include/asm/linkage.h
> +++ b/arch/powerpc/include/asm/linkage.h
> @@ -2,8 +2,12 @@
>   #ifndef _ASM_POWERPC_LINKAGE_H
>   #define _ASM_POWERPC_LINKAGE_H
>   
> +#include <linux/stringify.h>
>   #include <asm/types.h>
>   
> +#define __ALIGN			.align 2
> +#define __ALIGN_STR		__stringify(__ALIGN)
> +

I still can't see the added value of using __stringify() macro here. In 
order to use that macro you have to include linux/stringify.h . Usually 
we try to minimise the amount of headers required by other headers.

>   #ifdef CONFIG_PPC64_ELF_ABI_V1
>   #define cond_syscall(x) \
>   	asm ("\t.weak " #x "\n\t.set " #x ", sys_ni_syscall\n"		\

Hi Christophe,

On 29/08/22 18:56, Christophe Leroy wrote:
>
> Le 29/08/2022 à 07:52, Sathvika Vasireddy a écrit :
>> Powerpc instructions must be word-aligned. Currently,
>> there is an alignment of 16 bytes (by default), and it is
>> much more than what is required for powerpc (4 bytes).
>>
>> The default expansion of __ALIGN() macro is:
>> #define __ALIGN       .align 4,0x90
>>
>> Since Powerpc Linux does not require a 16 byte alignment,
>> override __ALIGN() and __ALIGN_STR() macros to use required
>> 4 byte alignment.
>>
>> Signed-off-by: Sathvika Vasireddy <sv@linux.ibm.com>
>> ---
>>    arch/powerpc/include/asm/linkage.h | 4 ++++
>>    1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/powerpc/include/asm/linkage.h b/arch/powerpc/include/asm/linkage.h
>> index b71b9582e754..8df88fe61438 100644
>> --- a/arch/powerpc/include/asm/linkage.h
>> +++ b/arch/powerpc/include/asm/linkage.h
>> @@ -2,8 +2,12 @@
>>    #ifndef _ASM_POWERPC_LINKAGE_H
>>    #define _ASM_POWERPC_LINKAGE_H
>>    
>> +#include <linux/stringify.h>
>>    #include <asm/types.h>
>>    
>> +#define __ALIGN			.align 2
>> +#define __ALIGN_STR		__stringify(__ALIGN)
>> +
> I still can't see the added value of using __stringify() macro here. In
> order to use that macro you have to include linux/stringify.h . Usually
> we try to minimise the amount of headers required by other headers.
Oh ok, makes sense to me. I'll wait for a day to see if there are any
other comments, and make this change as part of v3.

Thanks for reviewing!

- Sathvika