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

["Christopher M. Riedl" <cmr@codefail.de>]

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)

> Christophe
>
>
> > 
> > Signed-off-by: Christopher M. Riedl <cmr@codefail.de>
> > ---
> >   arch/powerpc/kernel/signal.h | 33 +++++++++++++++++++++++++++++++++
> >   1 file changed, 33 insertions(+)
> > 
> > diff --git a/arch/powerpc/kernel/signal.h b/arch/powerpc/kernel/signal.h
> > index 2559a681536e..e9aaeac0da37 100644
> > --- a/arch/powerpc/kernel/signal.h
> > +++ b/arch/powerpc/kernel/signal.h
> > @@ -53,6 +53,33 @@ unsigned long copy_ckfpr_from_user(struct task_struct *task, void __user *from);
> >   				&buf[i], label);\
> >   } while (0)
> >   
> > +#define unsafe_copy_fpr_from_user(task, from, label)	do {		\
> > +	struct task_struct *__t = task;					\
> > +	u64 __user *__f = (u64 __user *)from;				\
> > +	u64 buf[ELF_NFPREG];						\
> > +	int i;								\
> > +									\
> > +	unsafe_copy_from_user(buf, __f, ELF_NFPREG * sizeof(double),	\
> > +				label);					\
> > +	for (i = 0; i < ELF_NFPREG - 1; i++)				\
> > +		__t->thread.TS_FPR(i) = buf[i];				\
> > +	__t->thread.fp_state.fpscr = buf[i];				\
> > +} while (0)
> > +
> > +#define unsafe_copy_vsx_from_user(task, from, label)	do {		\
> > +	struct task_struct *__t = task;					\
> > +	u64 __user *__f = (u64 __user *)from;				\
> > +	u64 buf[ELF_NVSRHALFREG];					\
> > +	int i;								\
> > +									\
> > +	unsafe_copy_from_user(buf, __f,					\
> > +				ELF_NVSRHALFREG * sizeof(double),	\
> > +				label);					\
> > +	for (i = 0; i < ELF_NVSRHALFREG ; i++)				\
> > +		__t->thread.fp_state.fpr[i][TS_VSRLOWOFFSET] = buf[i];	\
> > +} while (0)
> > +
> > +
> >   #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
> >   #define unsafe_copy_ckfpr_to_user(to, task, label)	do {		\
> >   	struct task_struct *__t = task;					\
> > @@ -80,6 +107,10 @@ unsigned long copy_ckfpr_from_user(struct task_struct *task, void __user *from);
> >   	unsafe_copy_to_user(to, (task)->thread.fp_state.fpr,	\
> >   			    ELF_NFPREG * sizeof(double), label)
> >   
> > +#define unsafe_copy_fpr_from_user(task, from, label)		\
> > +	unsafe_copy_from_user((task)->thread.fp_state.fpr, from	\
> > +			    ELF_NFPREG * sizeof(double), label)
> > +
> >   static inline unsigned long
> >   copy_fpr_to_user(void __user *to, struct task_struct *task)
> >   {
> > @@ -115,6 +146,8 @@ copy_ckfpr_from_user(struct task_struct *task, void __user *from)
> >   #else
> >   #define unsafe_copy_fpr_to_user(to, task, label) do { } while (0)
> >   
> > +#define unsafe_copy_fpr_from_user(task, from, label) do { } while (0)
> > +
> >   static inline unsigned long
> >   copy_fpr_to_user(void __user *to, struct task_struct *task)
> >   {
> >