Re: [PATCH 03/20] arm64/fpsimd: signal: Clear PSTATE.SM when restoring FPSIMD frame only

[Mark Rutland <mark.rutland@arm.com>]

On Wed, May 07, 2025 at 01:46:45PM +0100, Will Deacon wrote:
> On Tue, May 06, 2025 at 04:25:06PM +0100, Mark Rutland wrote:
> > diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
> > index 48d3c0129dade..fdce1b856f498 100644
> > --- a/arch/arm64/kernel/signal.c
> > +++ b/arch/arm64/kernel/signal.c
> > @@ -280,6 +280,7 @@ static int restore_fpsimd_context(struct user_ctxs *user)
> >  	__get_user_error(fpsimd.fpcr, &(user->fpsimd->fpcr), err);
> >  
> >  	clear_thread_flag(TIF_SVE);
> > +	current->thread.svcr &= ~SVCR_SM_MASK;
> >  	current->thread.fp_type = FP_STATE_FPSIMD;
> 
> Hmm, I think we're preemptible here so do we need some compiler barriers
> to make sure that the context-switching code doesn't see these fields in
> an inconsistent state?

We avoid that problem by ensuring that the task's FPSIMD/SVE/SME state isn't
live on any CPU for the duration of the signal code, such that it is safe to
manipulate the saved state in preemptible context. Details below.

That was handled in one of the earlier patches in the arm64 for-next/sme-fixes
branch, queded as commit:

  929fa99b1215966f ("arm64/fpsimd: signal: Always save+flush state early")

As of that commit, in setup_rt_frame() we call
fpsimd_save_and_flush_current_state(), which does:

  get_cpu_fpsimd_context();
  fpsimd_save_user_state();
  fpsimd_flush_task_state(current);
  put_cpu_fpsimd_context();

That ensures (in a premption-safe way) that the tasks state has been saved
to memory, TIF_FOREIGN_FPSTATE is set, and that
task->thread.fpsimd_cpu==NR_CPUS. Thus on context switch:

* When switching away from this task, fpsimd_thread_switch() sees
  TIF_FOREIGN_FPSTATE is set, and doesn't manipulate the task's saved state.

* When switching to this task, fpsimd_thread_switch() doesn't find any existing
  state bound on the CPU, and sets TIF_FOREIGN_FPSTATE, without manipulating
  the task's saved state.

The task's state will only be relaoded onto the CPU when returning to
userspace, via fpsimd_restore_current_state().

Note that we changed fpsimd_update_current_state() to modify the saved state
without reloading it onto the CPU.

Mark.