[RFC PATCH v2 2/2] arm64: neon: Remove support for nested or hardirq kernel-mode NEON

[Dave Martin]

On Wed, May 24, 2017 at 08:22:18AM -0700, Ard Biesheuvel wrote:
> On 24 May 2017 at 07:42, Dave Martin <Dave.Martin at arm.com> wrote:
> > Support for kernel-mode NEON to be nested and/or used in hardirq
> > context adds significant complexity, and the benefits may be
> > marginal.  In practice, kernel-mode NEON is not used in hardirq
> > context, and is rarely used in softirq context (by certain mac80211
> > drivers).
> >
> > This patch implements an arm64 may_use_simd() function to allow
> > clients to check whether kernel-mode NEON is usable in the current
> > context, and simplifies kernel_neon_{begin,end}() to handle only
> > saving of the task FPSIMD state (if any).  Without nesting, there
> > is no other state to save.
> >
> > The partial fpsimd save/restore functions become redundant as a
> > result of these changes, so they are removed too.
> >
> > The save/restore model is changed to operate directly on
> > task_struct without additional percpu storage.  This simplifies the
> > code and saves a bit of memory, but means that softirqs must now be
> > disabled when manipulating the task fpsimd state from task context:
> > correspondingly, preempt_{en,dis}sable() calls are upgraded to
> > local_bh_{en,dis}able() as appropriate, and softirqs are also
> > disabled around the context switch code in fpsimd_thread_switch().
> > The will be some increase in softirq latency, but hardirqs should
> > be unaffected.
> >
> > These changes should make it easier to support kernel-mode NEON in
> > the presence of the Scalable Vector Extension in the future.
> >
> > Signed-off-by: Dave Martin <Dave.Martin at arm.com>

[...]

> > diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> > index 06da8ea..02023da 100644
> > --- a/arch/arm64/kernel/fpsimd.c
> > +++ b/arch/arm64/kernel/fpsimd.c
> > @@ -17,16 +17,20 @@

[...]

> > + * In order to allow softirq handlers to use FPSIMD, kernel_neon_begin() may
> > + * save the task's FPSIMD context back to task_struct from softirq context.
> > + * To prevent this from racing with the manipulation of the task's FPSIMD state
> > + * from task context and thereby corrupting the state, it is necessary to
> > + * protect any manipulation of a task's fpsimd_state or TIF_FOREIGN_FPSTATE
> > + * flag with local_bh_disable().
> > + *
> 
> Surely, this doesn't mean all kernel mode NEON should execute with
> bottom halves disabled? Because that is what this patch appears to be
> doing.

Dang, I was deliberately paranoid when updating
kernel_neon_{begin,end}(), but was _too_ paranoid.

softirq must be masked around the read-modify-write on
TIF_FOREIGN_FPSTATE and thread.fpsimd_state: from kernel_neon_begin() to
kernel_neon_end() we only need to disable preemption, to hide the lack
of context switch machinery for kernel-mode NEON state in task context
(as at present).

I think they should do:

[...]

> > +void kernel_neon_begin(void)
> >  {
> >         if (WARN_ON(!system_supports_fpsimd()))
> >                 return;

[...]

> > +       BUG_ON(!may_use_simd());
> > +
> > +       local_bh_disable();
> > +
> > +       __this_cpu_write(kernel_neon_busy, true);
> > +
> > +       /*
> > +        * If there is unsaved task fpsimd state in the CPU, save it
> > +        * and invalidate the copy stored in the fpsimd regs:
> > +        */
> > +       if (current->mm && !test_and_set_thread_flag(TIF_FOREIGN_FPSTATE)) {
> > +               fpsimd_save_state(&current->thread.fpsimd_state);
> >                 this_cpu_write(fpsimd_last_state, NULL);
> >         }

	preempt_disable();

	local_bh_enable();

> >  }
> > -EXPORT_SYMBOL(kernel_neon_begin_partial);
> > +EXPORT_SYMBOL(kernel_neon_begin);
> >
> > +/*
> > + * kernel_neon_end(): give the CPU FPSIMD registers back to the current task
> > + *
> > + * Must be called from a context in which kernel_neon_begin() was previously
> > + * called, with no call to kernel_neon_end() in the meantime.
> > + *
> > + * The caller must not use the FPSIMD registers after this function is called,
> > + * unless kernel_neon_begin() is called again in the meantime.
> > + */
> >  void kernel_neon_end(void)
> >  {
> > +       bool busy;
> > +
> >         if (!system_supports_fpsimd())
> >                 return;

[...]

> > +
> > +       busy = __this_cpu_xchg(kernel_neon_busy, false);
> > +       WARN_ON(!busy); /* No matching kernel_neon_begin()? */
> > +

	preempt_enable();

> >  }
> >  EXPORT_SYMBOL(kernel_neon_end);

Make sense?

Cheers
---Dave