[PATCH v2 1/9] s390/percpu: Provide arch_raw_cpu_ptr()

Heiko Carstens
2026-03-19T12:05:15Z

Provide an s390 specific arch_raw_cpu_ptr() implementation which avoids the
detour over get_lowcore() to get the lowcore pointer. The inline assembly
is implemented with an alternative so that relocated lowcore (percpu offset
is at a different address) is handled correctly.

This turns code like this

102f78: a7 39 00 00 lghi %r3,0
102f7c: e3 20 33 b8 00 08 ag %r2,952(%r3)

which adds the percpu offset to register r2 into a single instruction

102f7c: e3 20 33 b8 00 08 ag %r2,952(%r0)

and also avoids the need of a base register, thus reducing register
pressure.

With defconfig bloat-o-meter -t provides this result:

add/remove: 12/26 grow/shrink: 183/3391 up/down: 14880/-41950 (-27070)

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
---
arch/s390/include/asm/percpu.h | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)

diff --git a/arch/s390/include/asm/percpu.h b/arch/s390/include/asm/percpu.h
index 5899f57f17d1..b18a96f3a334 100644
--- a/arch/s390/include/asm/percpu.h
+++ b/arch/s390/include/asm/percpu.h
@@ -12,6 +12,24 @@
*/
#define __my_cpu_offset get_lowcore()->percpu_offset

+#define arch_raw_cpu_ptr(_ptr) \
+({ \
+ unsigned long lc_percpu, tcp_ptr__; \
+ \
+ tcp_ptr__ = (__force unsigned long)(_ptr); \
+ lc_percpu = offsetof(struct lowcore, percpu_offset); \
+ asm_inline volatile( \
+ ALTERNATIVE("ag %[__ptr__],%[offzero](%%r0)\n", \
+ "ag %[__ptr__],%[offalt](%%r0)\n", \
+ ALT_FEATURE(MFEATURE_LOWCORE)) \
+ : [__ptr__] "+d" (tcp_ptr__) \
+ : [offzero] "i" (lc_percpu), \
+ [offalt] "i" (lc_percpu + LOWCORE_ALT_ADDRESS), \
+ "m" (((struct lowcore *)0)->percpu_offset) \
+ : "cc"); \
+ (TYPEOF_UNQUAL(*(_ptr)) __force __kernel *)tcp_ptr__; \
+})
+
/*
* We use a compare-and-swap loop since that uses less cpu cycles than
* disabling and enabling interrupts like the generic variant would do.
--
2.51.0