powerpc/signal: Add helper function to fetch quad word aligned pointer

This patch adds one helper function 'sigcontext_vmx_regs' which computes
quad word aligned pointer for 'vmx_reserve' array element in sigcontext
structure making the code more readable.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
[mpe: Reword comment and fix build for CONFIG_ALTIVEC=n]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/signal_64.c

@@ -73,6 +73,19 @@ static const char fmt32[] = KERN_INFO \
 static const char fmt64[] = KERN_INFO \
 	"%s[%d]: bad frame in %s: %016lx nip %016lx lr %016lx\n";
 
+/*
+ * This computes a quad word aligned pointer inside the vmx_reserve array
+ * element. For historical reasons sigcontext might not be quad word aligned,
+ * but the location we write the VMX regs to must be. See the comment in
+ * sigcontext for more detail.
+ */
+#ifdef CONFIG_ALTIVEC
+static elf_vrreg_t __user *sigcontext_vmx_regs(struct sigcontext __user *sc)
+{
+	return (elf_vrreg_t __user *) (((unsigned long)sc->vmx_reserve + 15) & ~0xful);
+}
+#endif
+
 /*
  * Set up the sigcontext for the signal frame.
  */
@@ -90,7 +103,7 @@ static long setup_sigcontext(struct sigcontext __user *sc, struct pt_regs *regs,
 	 * v_regs pointer or not
 	 */
 #ifdef CONFIG_ALTIVEC
-	elf_vrreg_t __user *v_regs = (elf_vrreg_t __user *)(((unsigned long)sc->vmx_reserve + 15) & ~0xful);
+	elf_vrreg_t __user *v_regs = sigcontext_vmx_regs(sc);
 #endif
 	unsigned long msr = regs->msr;
 	long err = 0;
@@ -181,10 +194,8 @@ static long setup_tm_sigcontexts(struct sigcontext __user *sc,
 	 * v_regs pointer or not.
 	 */
 #ifdef CONFIG_ALTIVEC
-	elf_vrreg_t __user *v_regs = (elf_vrreg_t __user *)
-		(((unsigned long)sc->vmx_reserve + 15) & ~0xful);
-	elf_vrreg_t __user *tm_v_regs = (elf_vrreg_t __user *)
-		(((unsigned long)tm_sc->vmx_reserve + 15) & ~0xful);
+	elf_vrreg_t __user *v_regs = sigcontext_vmx_regs(sc);
+	elf_vrreg_t __user *tm_v_regs = sigcontext_vmx_regs(tm_sc);
 #endif
 	unsigned long msr = regs->msr;
 	long err = 0;