[PATCH v4 02/10] powerpc/signal: Add unsafe_copy_{vsx, fpr}_from_user()

[David Laight]

From: Christopher M. Riedl
> Sent: 01 February 2021 16:55
...
> > > > > +	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)
> >
> > On further reflection, since you immediately loop through the buffer
> > why not just use user_access_begin() and unsafe_get_user() in the loop.
> 
> Christophe had suggested this a few revisions ago as well. When I tried
> this approach, the signal handling performance took a pretty big hit:
> https://lists.ozlabs.org/pipermail/linuxppc-dev/2020-October/219351.html
> 
> I included some numbers on v3 as well but decided to drop the approach
> altogether for this one since it just didn't seem worth the hit.

Was that using unsafe_get_user (which relies on user_access_begin()
having 'opened up' user accesses) or just get_user() that does
it for every access?

The former should be ok, the latter will be horrid.

	David

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