s390/preempt: Optimize preempt_count()

Provide an inline assembly using alternatives to avoid the need of
a base register when reading preempt_count() from lowcore. Use the
LLGT instruction, which reads only the least significant 31 bits of
preempt_count. This masks out the encoded PREEMPT_NEED_RESCHED bit.

Generated code is changed from

000000000046e5d0 <vfree>:
  46e5d0:       c0 04 00 00 00 00       jgnop   46e5d0 <vfree>
  46e5d6:       a7 39 00 00             lghi    %r3,0
  46e5da:       58 10 33 a8             l       %r1,936(%r3)
  46e5de:       c0 1b 00 ff ff 00       nilf    %r1,16776960
  46e5e4:       a7 74 00 11             jne     46e606 <vfree+0x36>

to something like this:

000000000046e5d0 <vfree>:
  46e5d0:       c0 04 00 00 00 00       jgnop   46e5d0 <vfree>
  46e5d6:       e3 10 03 a8 00 17       llgt    %r1,936
  46e5dc:       ec 41 28 b7 00 55       risbgz  %r4,%r1,40,55
  46e5e2:       a7 74 00 0f             jne     46e600 <vfree+0x30>

Overall savings are only 82 bytes according to bloat-o-meter. This
is because of different inlining decisions, and there aren't many
preempt_count() users in the kernel.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

diff --git a/arch/s390/include/asm/preempt.h b/arch/s390/include/asm/preempt.h
index 6ccd033acfe52f5044e8b51623c700856506df92..6d6a28bee4e70abfc7d22d317dede7a8b12e1835 100644 (file)
--- a/arch/s390/include/asm/preempt.h
+++ b/arch/s390/include/asm/preempt.h
@@ -8,7 +8,10 @@
 #include <asm/cmpxchg.h>
 #include <asm/march.h>
 
-/* We use the MSB mostly because its available */
+/*
+ * Use MSB so it is possible to read preempt_count with LLGT which
+ * reads the least significant 31 bits with a single instruction.
+ */
 #define PREEMPT_NEED_RESCHED   0x80000000
 
 /*
@@ -23,7 +26,20 @@
  */
 static __always_inline int preempt_count(void)
 {
-       return READ_ONCE(get_lowcore()->preempt_count) & ~PREEMPT_NEED_RESCHED;
+       unsigned long lc_preempt, count;
+
+       BUILD_BUG_ON(sizeof_field(struct lowcore, preempt_count) != sizeof(int));
+       lc_preempt = offsetof(struct lowcore, preempt_count);
+       /* READ_ONCE(get_lowcore()->preempt_count) & ~PREEMPT_NEED_RESCHED */
+       asm_inline(
+               ALTERNATIVE("llgt       %[count],%[offzero](%%r0)\n",
+                           "llgt       %[count],%[offalt](%%r0)\n",
+                           ALT_FEATURE(MFEATURE_LOWCORE))
+               : [count] "=d" (count)
+               : [offzero] "i" (lc_preempt),
+                 [offalt] "i" (lc_preempt + LOWCORE_ALT_ADDRESS),
+                 "m" (((struct lowcore *)0)->preempt_count));
+       return count;
 }
 
 static __always_inline void preempt_count_set(int pc)