[RFC PATCH 13/29] arm64/sve: Basic support for KERNEL_MODE_NEON

[Catalin Marinas]

On Mon, Nov 28, 2016 at 11:47:26AM +0000, Dave P Martin wrote:
> On Sat, Nov 26, 2016 at 11:30:42AM +0000, Catalin Marinas wrote:
> > On Fri, Nov 25, 2016 at 08:45:02PM +0000, Ard Biesheuvel wrote:
> > > On 25 November 2016 at 19:39, Dave Martin <Dave.Martin at arm.com> wrote:
> > > > --- a/arch/arm64/kernel/fpsimd.c
> > > > +++ b/arch/arm64/kernel/fpsimd.c
> > > > @@ -282,11 +282,26 @@ static DEFINE_PER_CPU(struct fpsimd_partial_state, softirq_fpsimdstate);
> > > >   */
> > > >  void kernel_neon_begin_partial(u32 num_regs)
> > > >  {
> > > > +       preempt_disable();
> > > > +
> > > > +       /*
> > > > +        * For now, we have no special storage for SVE registers in
> > > > +        * interrupt context, so always save the userland SVE state
> > > > +        * if there is any, even for interrupts.
> > > > +        */
> > > > +       if (IS_ENABLED(CONFIG_ARM64_SVE) && (elf_hwcap & HWCAP_SVE) &&
> > > > +           current->mm &&
> > > > +           !test_and_set_thread_flag(TIF_FOREIGN_FPSTATE)) {
> > > > +               fpsimd_save_state(&current->thread.fpsimd_state);
> > > > +               this_cpu_write(fpsimd_last_state, NULL);
> > > > +       }
> > > > +
> > > 
> > > I am having trouble understanding why we need all of this if we don't
> > > support SVE in the kernel. Could you elaborate?
> > 
> > Dave knows all the details but a reason is that touching a Neon register
> > zeros the upper SVE state in the same vector register. So we can't
> > safely save/restore just the Neon part without corrupting the SVE state.
> 
> This is right -- this also means that EFI services can trash the upper
> bits of an SVE vector register (as a side-effect of FPSIMD/NEON usage).
> 
> It's overkill to save/restore absolutely everything -- I ignore num_regs
> for example -- but I wanted to keep things as simple as possible
> initially.

Without looking at your patches in detail, could we mandate in the ABI
that the SVE state is lost on the user/kernel syscall boundary? I guess
even for the PCS, the SVE state is caller-saved, so there shouldn't be
an additional cost to user. On interrupts, however, we'd have to
preserve the SVE state but if we do this on entry/exit points, the
kernel_neon_*() functions would not have to deal with any SVE state (and
even ignore it completely if in interrupt).

BTW, we will need an SVE ABI document in Documentation/arm64/ to specify
the requirements for syscall and sigcontext modifications.

-- 
Catalin