[PATCH v3 2/2] ARM: vfp: clear fpscr length and stride bits on entry to sig handler

Hi Dave,

Thanks for looking at this.

On Fri, Feb 03, 2012 at 11:12:09AM +0000, Dave Martin wrote:

On Wed, Feb 01, 2012 at 02:03:12PM +0000, Will Deacon wrote:

quoted

diff --git a/arch/arm/vfp/vfpmodule.c b/arch/arm/vfp/vfpmodule.c
index 3e35e35..49cf328 100644
--- a/arch/arm/vfp/vfpmodule.c
+++ b/arch/arm/vfp/vfpmodule.c

@@ -562,6 +562,21 @@ int vfp_preserve_user_hwstate(struct user_vfp __user *ufp,
 
 	if (err)
 		return -EFAULT;
+
+	/* Ensure that VFP is disabled. */
+	vfp_flush_hwstate(thread);
+
+	/*
+	 * As per the PCS, clear the length and stride bits before entry
+	 * to the signal handler.
+	 */
+	hwstate->fpscr &= ~(FPSCR_LENGTH_MASK | FPSCR_STRIDE_MASK);
+
+	/*
+	 * Disable VFP in the hwstate so that we can detect if it was
+	 * used by the signal handler.
+	 */
+	hwstate->fpexc &= ~FPEXC_EN;
 	return 0;
 }

^ 
These additions are nothing to do with preserving the VFP state.
Instead, they set up a clean signal frame, under the assumption that the
interrupted thread's state has already been preserved.

Would this be more cleanly factored out as a separate function,
something like vfp_setup_sigframe(), so:

setup_sigframe() {
	/* ... */

	vfp_preserve_user_hwstate();
	vfp_setup_sigframe();
}

...?

The whole reason for moving this code out of signal.c was to avoid polluting
that file with VFP-related code. I don't want to turn the problem on its
head and end up with signal-related code in vfpmodule.c (so yes, I'll fix those
comments :). setup_sigframe certainly sounds out of place here.

Alternatively, if this is an abstraction too far, and the "preserve" and
"setup sigframe" actions will never make sense independently of each
other, you could just rename the function to something clearer, like
vfp_user_preserve_setup_sigframe().

Indeed, this comes down to my inability to think of a decent function name.
I'd like to avoid the _sigframe bit, so I'll have a think about some better
terminology. I could try running a competition.

Expect a v4 when inspiration strikes.

Will