On Wed, Sep 13, 2017 at 10:26:05AM -0700, Catalin Marinas wrote:

On Thu, Aug 31, 2017 at 06:00:43PM +0100, Dave P Martin wrote:

quoted

+el0_sve_acc:
+	/*
+	 * Scalable Vector Extension access
+	 */
+	enable_dbg
+	ct_user_exit
+	mov	x0, x25
+	mov	x1, sp
+	bl	do_sve_acc
+	b	ret_to_user

I think do_sve_acc() runs with interrupts disabled. We may have some
high latency for large SVE states.

Historically I wanted to play safe.

I meant to change this to enable_dbg_and_irq now, but it looks like I
forgot to do it.

I'll double-check that do_sve_acc() does nothing that makes it unsafe to
enable irqs, but it should be OK.  It's certainly _intended_ to be
irq-safe.

quoted

+/*
+ * Trapped SVE access
+ */
+void do_sve_acc(unsigned int esr, struct pt_regs *regs)
+{
+	/* Even if we chose not to use SVE, the hardware could still trap: */
+	if (unlikely(!system_supports_sve()) || WARN_ON(is_compat_task())) {
+		force_signal_inject(SIGILL, ILL_ILLOPC, regs, 0);
+		return;
+	}
+
+	task_fpsimd_save();
+
+	sve_alloc(current);
+	fpsimd_to_sve(current);
+	if (test_and_set_thread_flag(TIF_SVE))
+		WARN_ON(1); /* SVE access shouldn't have trapped */
+
+	task_fpsimd_load();
+}

When this function is entered, do we expect TIF_SVE to always be
cleared? It's worth adding a comment on the expected conditions. If

Yes, and this is required for correctness, as you observe.

I had a BUG_ON() here which I removed, but it makes sense to add a
comment to capture the precondition here, and how it is satisfied.

that's the case, task_fpsimd_save() would only save the FPSIMD state
which is fine. However, you subsequently transfer the FPSIMD state to
SVE, set TIF_SVE and restore the full SVE state. If we don't care about
the SVE state here, can we call task_fpsimd_load() *before* setting
TIF_SVE?

There should be no way to reach this code with TIF_SVE set, unless
task_fpsimd_load() sets the CPACR trap bit wrongly, or the hardware is
broken -- either of which is a bug.

I may as well have confused myself with the state bouncing between
FPSIMD and SVE (more reasons to document the data flow better ;)).

Agreed, I think this does need more explanation...  I'm currently
trying to figure out the best way to describe it.

Cheers
---Dave