[PATCH v2 3/6] riscv: Add Zawrs support for spinlocks

Andrew Jones
2024-04-19T13:53:29Z

From: Christoph Müllner <christoph.muellner@vrull.eu>

RISC-V code uses the generic ticket lock implementation, which calls
the macros smp_cond_load_relaxed() and smp_cond_load_acquire().
Introduce a RISC-V specific implementation of smp_cond_load_relaxed()
which applies WRS.NTO of the Zawrs extension in order to reduce power
consumption while waiting and allows hypervisors to enable guests to
trap while waiting. smp_cond_load_acquire() doesn't need a RISC-V
specific implementation as the generic implementation is based on
smp_cond_load_relaxed() and smp_acquire__after_ctrl_dep() sufficiently
provides the acquire semantics.

This implementation is heavily based on Arm's approach which is the
approach Andrea Parri also suggested.

The Zawrs specification can be found here:
https://github.com/riscv/riscv-zawrs/blob/main/zawrs.adoc

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Co-developed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
---
arch/riscv/Kconfig | 13 ++++++++
arch/riscv/include/asm/barrier.h | 45 ++++++++++++++++++---------
arch/riscv/include/asm/cmpxchg.h | 51 +++++++++++++++++++++++++++++++
arch/riscv/include/asm/hwcap.h | 1 +
arch/riscv/include/asm/insn-def.h | 2 ++
arch/riscv/kernel/cpufeature.c | 1 +
6 files changed, 98 insertions(+), 15 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 7427d8088337..34bbe6b70546 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -578,6 +578,19 @@ config RISCV_ISA_V_PREEMPTIVE
preemption. Enabling this config will result in higher memory
consumption due to the allocation of per-task's kernel Vector context.

+config RISCV_ISA_ZAWRS
+ bool "Zawrs extension support for more efficient busy waiting"
+ depends on RISCV_ALTERNATIVE
+ default y
+ help
+ The Zawrs extension defines instructions to be used in polling loops
+ which allow a hart to enter a low-power state or to trap to the
+ hypervisor while waiting on a store to a memory location. Enable the
+ use of these instructions in the kernel when the Zawrs extension is
+ detected at boot.
+
+ If you don't know what to do here, say Y.
+
config TOOLCHAIN_HAS_ZBB
bool
default y
diff --git a/arch/riscv/include/asm/barrier.h b/arch/riscv/include/asm/barrier.h
index 880b56d8480d..e1d9bf1deca6 100644
--- a/arch/riscv/include/asm/barrier.h
+++ b/arch/riscv/include/asm/barrier.h
@@ -11,6 +11,7 @@
#define _ASM_RISCV_BARRIER_H

#ifndef __ASSEMBLY__
+#include <asm/cmpxchg.h>
#include <asm/fence.h>

#define nop() __asm__ __volatile__ ("nop")
@@ -28,21 +29,6 @@
#define __smp_rmb() RISCV_FENCE(r, r)
#define __smp_wmb() RISCV_FENCE(w, w)

-#define __smp_store_release(p, v) \
-do { \
- compiletime_assert_atomic_type(*p); \
- RISCV_FENCE(rw, w); \
- WRITE_ONCE(*p, v); \
-} while (0)
-
-#define __smp_load_acquire(p) \
-({ \
- typeof(*p) ___p1 = READ_ONCE(*p); \
- compiletime_assert_atomic_type(*p); \
- RISCV_FENCE(r, rw); \
- ___p1; \
-})
-
/*
* This is a very specific barrier: it's currently only used in two places in
* the kernel, both in the scheduler. See include/linux/spinlock.h for the two
@@ -70,6 +56,35 @@ do { \
*/
#define smp_mb__after_spinlock() RISCV_FENCE(iorw, iorw)

+#define __smp_store_release(p, v) \
+do { \
+ compiletime_assert_atomic_type(*p); \
+ RISCV_FENCE(rw, w); \
+ WRITE_ONCE(*p, v); \
+} while (0)
+
+#define __smp_load_acquire(p) \
+({ \
+ typeof(*p) ___p1 = READ_ONCE(*p); \
+ compiletime_assert_atomic_type(*p); \
+ RISCV_FENCE(r, rw); \
+ ___p1; \
+})
+
+#ifdef CONFIG_RISCV_ISA_ZAWRS
+#define smp_cond_load_relaxed(ptr, cond_expr) ({ \
+ typeof(ptr) __PTR = (ptr); \
+ __unqual_scalar_typeof(*ptr) VAL; \
+ for (;;) { \
+ VAL = READ_ONCE(*__PTR); \
+ if (cond_expr) \
+ break; \
+ __cmpwait_relaxed(ptr, VAL); \
+ } \
+ (typeof(*ptr))VAL; \
+})
+#endif
+
#include <asm-generic/barrier.h>

#endif /* __ASSEMBLY__ */
diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
index 2fee65cc8443..0926ac7f4ca6 100644
--- a/arch/riscv/include/asm/cmpxchg.h
+++ b/arch/riscv/include/asm/cmpxchg.h
@@ -8,7 +8,10 @@

#include <linux/bug.h>

+#include <asm/alternative-macros.h>
#include <asm/fence.h>
+#include <asm/hwcap.h>
+#include <asm/insn-def.h>

#define __xchg_relaxed(ptr, new, size) \
({ \
@@ -359,4 +362,52 @@
arch_cmpxchg_relaxed((ptr), (o), (n)); \
})

+#ifdef CONFIG_RISCV_ISA_ZAWRS
+static __always_inline void __cmpwait(volatile void *ptr,
+ unsigned long val,
+ int size)
+{
+ unsigned long tmp;
+
+ asm goto(ALTERNATIVE("j %l[no_zawrs]", "nop",
+ 0, RISCV_ISA_EXT_ZAWRS, 1)
+ : : : : no_zawrs);
+
+ switch (size) {
+ case 4:
+ asm volatile(
+ " lr.w %0, %1\n"
+ " xor %0, %0, %2\n"
+ " bnez %0, 1f\n"
+ ZAWRS_WRS_NTO "\n"
+ "1:"
+ : "=&r" (tmp), "+A" (*(u32 *)ptr)
+ : "r" (val));
+ break;
+#if __riscv_xlen == 64
+ case 8:
+ asm volatile(
+ " lr.d %0, %1\n"
+ " xor %0, %0, %2\n"
+ " bnez %0, 1f\n"
+ ZAWRS_WRS_NTO "\n"
+ "1:"
+ : "=&r" (tmp), "+A" (*(u64 *)ptr)
+ : "r" (val));
+ break;
+#endif
+ default:
+ BUILD_BUG();
+ }
+
+ return;
+
+no_zawrs:
+ asm volatile(RISCV_PAUSE : : : "memory");
+}
+
+#define __cmpwait_relaxed(ptr, val) \
+ __cmpwait((ptr), (unsigned long)(val), sizeof(*(ptr)))
+#endif
+
#endif /* _ASM_RISCV_CMPXCHG_H */
diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
index e17d0078a651..5b358c3cf212 100644
--- a/arch/riscv/include/asm/hwcap.h
+++ b/arch/riscv/include/asm/hwcap.h
@@ -81,6 +81,7 @@
#define RISCV_ISA_EXT_ZTSO 72
#define RISCV_ISA_EXT_ZACAS 73
#define RISCV_ISA_EXT_XANDESPMU 74
+#define RISCV_ISA_EXT_ZAWRS 75

#define RISCV_ISA_EXT_XLINUXENVCFG 127

diff --git a/arch/riscv/include/asm/insn-def.h b/arch/riscv/include/asm/insn-def.h
index 64dffaa21bfa..9a913010cdd9 100644
--- a/arch/riscv/include/asm/insn-def.h
+++ b/arch/riscv/include/asm/insn-def.h
@@ -197,5 +197,7 @@
RS1(base), SIMM12(4))

#define RISCV_PAUSE ".4byte 0x100000f"
+#define ZAWRS_WRS_NTO ".4byte 0x00d00073"
+#define ZAWRS_WRS_STO ".4byte 0x01d00073"

#endif /* __ASM_INSN_DEF_H */
diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 3ed2359eae35..02de9eaa3f42 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -257,6 +257,7 @@ const struct riscv_isa_ext_data riscv_isa_ext[] = {
__RISCV_ISA_EXT_DATA(zihintpause, RISCV_ISA_EXT_ZIHINTPAUSE),
__RISCV_ISA_EXT_DATA(zihpm, RISCV_ISA_EXT_ZIHPM),
__RISCV_ISA_EXT_DATA(zacas, RISCV_ISA_EXT_ZACAS),
+ __RISCV_ISA_EXT_DATA(zawrs, RISCV_ISA_EXT_ZAWRS),
__RISCV_ISA_EXT_DATA(zfa, RISCV_ISA_EXT_ZFA),
__RISCV_ISA_EXT_DATA(zfh, RISCV_ISA_EXT_ZFH),
__RISCV_ISA_EXT_DATA(zfhmin, RISCV_ISA_EXT_ZFHMIN),
--
2.44.0