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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