Re: [PATCH 2/8] powerpc/signal: Add unsafe_copy_{vsx, fpr}_from_user()

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 20/10/2020 à 04:01, Christopher M. Riedl a écrit :
> On Fri Oct 16, 2020 at 10:48 AM CDT, Christophe Leroy wrote:
>>
>>
>> Le 15/10/2020 à 17:01, Christopher M. Riedl a écrit :
>>> Reuse the "safe" implementation from signal.c except for calling
>>> unsafe_copy_from_user() to copy into a local buffer. Unlike the
>>> unsafe_copy_{vsx,fpr}_to_user() functions the "copy from" functions
>>> cannot use unsafe_get_user() directly to bypass the local buffer since
>>> doing so significantly reduces signal handling performance.
>>
>> Why can't the functions use unsafe_get_user(), why does it significantly
>> reduces signal handling
>> performance ? How much significant ? I would expect that not going
>> through an intermediate memory
>> area would be more efficient
>>
> 
> Here is a comparison, 'unsafe-signal64-regs' avoids the intermediate buffer:
> 
> 	|                      | hash   | radix  |
> 	| -------------------- | ------ | ------ |
> 	| linuxppc/next        | 289014 | 158408 |
> 	| unsafe-signal64      | 298506 | 253053 |
> 	| unsafe-signal64-regs | 254898 | 220831 |
> 
> I have not figured out the 'why' yet. As you mentioned in your series,
> technically calling __copy_tofrom_user() is overkill for these
> operations. The only obvious difference between unsafe_put_user() and
> unsafe_get_user() is that we don't have asm-goto for the 'get' variant.
> Instead we wrap with unsafe_op_wrap() which inserts a conditional and
> then goto to the label.
> 
> Implemenations:
> 
> 	#define unsafe_copy_fpr_from_user(task, from, label)   do {            \
> 	       struct task_struct *__t = task;                                 \
> 	       u64 __user *buf = (u64 __user *)from;                           \
> 	       int i;                                                          \
> 									       \
> 	       for (i = 0; i < ELF_NFPREG - 1; i++)                            \
> 		       unsafe_get_user(__t->thread.TS_FPR(i), &buf[i], label); \
> 	       unsafe_get_user(__t->thread.fp_state.fpscr, &buf[i], label);    \
> 	} while (0)
> 
> 	#define unsafe_copy_vsx_from_user(task, from, label)   do {            \
> 	       struct task_struct *__t = task;                                 \
> 	       u64 __user *buf = (u64 __user *)from;                           \
> 	       int i;                                                          \
> 									       \
> 	       for (i = 0; i < ELF_NVSRHALFREG ; i++)                          \
> 		       unsafe_get_user(__t->thread.fp_state.fpr[i][TS_VSRLOWOFFSET], \
> 				       &buf[i], label);                        \
> 	} while (0)
> 

Do you have CONFIG_PROVE_LOCKING or CONFIG_DEBUG_ATOMIC_SLEEP enabled in your config ?

If yes, could you try together with the patch from Alexey 
https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20210204121612.32721-1-aik@ozlabs.ru/ ?

Thanks
Christophe