[PATCH v4 07/13] riscv: Implement arch_cmpxchg128() using Zacas

Alexandre Ghiti
2024-07-31T07:31:18Z

Now that Zacas is supported in the kernel, let's use the double word
atomic version of amocas to improve the SLUB allocator.

Note that we have to select fixed registers, otherwise gcc fails to pick
even registers and then produces a reserved encoding which fails to
assemble.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
---
arch/riscv/Kconfig | 1 +
arch/riscv/include/asm/cmpxchg.h | 38 ++++++++++++++++++++++++++++++++
2 files changed, 39 insertions(+)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 212ec2aab389..ef55ab94027e 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -111,6 +111,7 @@ config RISCV
select GENERIC_VDSO_TIME_NS if HAVE_GENERIC_VDSO
select HARDIRQS_SW_RESEND
select HAS_IOPORT if MMU
+ select HAVE_ALIGNED_STRUCT_PAGE
select HAVE_ARCH_AUDITSYSCALL
select HAVE_ARCH_HUGE_VMALLOC if HAVE_ARCH_HUGE_VMAP
select HAVE_ARCH_HUGE_VMAP if MMU && 64BIT
diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
index 391730367213..ce9613516bbb 100644
--- a/arch/riscv/include/asm/cmpxchg.h
+++ b/arch/riscv/include/asm/cmpxchg.h
@@ -276,6 +276,44 @@
arch_cmpxchg_release((ptr), (o), (n)); \
})

+#if defined(CONFIG_64BIT) && defined(CONFIG_RISCV_ISA_ZACAS)
+
+#define system_has_cmpxchg128() riscv_has_extension_unlikely(RISCV_ISA_EXT_ZACAS)
+
+union __u128_halves {
+ u128 full;
+ struct {
+ u64 low, high;
+ };
+};
+
+#define __arch_cmpxchg128(p, o, n, cas_sfx) \
+({ \
+ __typeof__(*(p)) __o = (o); \
+ union __u128_halves __hn = { .full = (n) }; \
+ union __u128_halves __ho = { .full = (__o) }; \
+ register unsigned long t1 asm ("t1") = __hn.low; \
+ register unsigned long t2 asm ("t2") = __hn.high; \
+ register unsigned long t3 asm ("t3") = __ho.low; \
+ register unsigned long t4 asm ("t4") = __ho.high; \
+ \
+ __asm__ __volatile__ ( \
+ " amocas.q" cas_sfx " %0, %z3, %2" \
+ : "+&r" (t3), "+&r" (t4), "+A" (*(p)) \
+ : "rJ" (t1), "rJ" (t2) \
+ : "memory"); \
+ \
+ ((u128)t4 << 64) | t3; \
+})
+
+#define arch_cmpxchg128(ptr, o, n) \
+ __arch_cmpxchg128((ptr), (o), (n), ".aqrl")
+
+#define arch_cmpxchg128_local(ptr, o, n) \
+ __arch_cmpxchg128((ptr), (o), (n), "")
+
+#endif /* CONFIG_64BIT && CONFIG_RISCV_ISA_ZACAS */
+
#ifdef CONFIG_RISCV_ISA_ZAWRS
/*
* Despite wrs.nto being "WRS-with-no-timeout", in the absence of changes to
--
2.39.2