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Subject[RFC PATCH v2] riscv: Add Zawrs support for spinlocks
Date
From: Christoph Müllner <christoph.muellner@vrull.eu>

The current RISC-V code uses the generic ticket lock implementation,
that calls the macros smp_cond_load_relaxed() and smp_cond_load_acquire().
Currently, RISC-V uses the generic implementation of these macros.
This patch introduces a RISC-V specific implementation, of these
macros, that peels off the first loop iteration and modifies the waiting
loop such, that it is possible to use the WRS.STO instruction of the Zawrs
ISA extension to stall the CPU.

The resulting implementation of smp_cond_load_*() will only work for
32-bit or 64-bit types for RV64 and 32-bit types for RV32.
This is caused by the restrictions of the LR instruction (RISC-V only
has LR.W and LR.D). Compiler assertions guard this new restriction.

This patch uses the existing RISC-V ISA extension framework
to detect the presents of Zawrs at run-time.
If available a NOP instruction will be replaced by WRS.NTO or WRS.STO.

The whole mechanism is gated by Kconfig setting, which defaults to Y.

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

Note, that the Zawrs extension is not frozen or ratified yet.
Therefore this patch is an RFC and not intended to get merged.

Changes since v1:
* Adding "depends on !XIP_KERNEL" to RISCV_ISA_ZAWRS
* Fixing type checking code in __smp_load_reserved*
* Adjustments according to the specification change

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
---
arch/riscv/Kconfig | 11 ++++
arch/riscv/include/asm/barrier.h | 92 ++++++++++++++++++++++++++++
arch/riscv/include/asm/errata_list.h | 19 +++++-
arch/riscv/include/asm/hwcap.h | 3 +-
arch/riscv/kernel/cpu.c | 1 +
arch/riscv/kernel/cpufeature.c | 13 ++++
6 files changed, 136 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 32ffef9f6e5b..9d40569237c9 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -358,6 +358,17 @@ config RISCV_ISA_C

If you don't know what to do here, say Y.

+config RISCV_ISA_ZAWRS
+ bool "Zawrs extension support"
+ depends on !XIP_KERNEL
+ select RISCV_ALTERNATIVE
+ default y
+ help
+ Adds support to dynamically detect the presence of the Zawrs extension
+ (wait for reservation set) and enable its usage.
+
+ If you don't know what to do here, say Y.
+
config RISCV_ISA_SVPBMT
bool "SVPBMT extension support"
depends on 64BIT && MMU
diff --git a/arch/riscv/include/asm/barrier.h b/arch/riscv/include/asm/barrier.h
index d0e24aaa2aa0..1f9628aaa7cb 100644
--- a/arch/riscv/include/asm/barrier.h
+++ b/arch/riscv/include/asm/barrier.h
@@ -12,6 +12,8 @@

#ifndef __ASSEMBLY__

+#include <asm/errata_list.h>
+
#define nop() __asm__ __volatile__ ("nop")

#define RISCV_FENCE(p, s) \
@@ -42,6 +44,64 @@ do { \
___p1; \
})

+#if __riscv_xlen == 64
+
+#define __riscv_lrsc_word(t) \
+ (sizeof(t) == sizeof(int) || \
+ sizeof(t) == sizeof(long))
+
+#elif __riscv_xlen == 32
+
+#define __riscv_lrsc_word(t) \
+ (sizeof(t) == sizeof(int))
+
+#else
+#error "Unexpected __riscv_xlen"
+#endif /* __riscv_xlen */
+
+#define compiletime_assert_atomic_lrsc_type(t) \
+ compiletime_assert(__riscv_lrsc_word(t), \
+ "Need type compatible with LR/SC instructions.")
+
+#define ___smp_load_reservedN(pfx, ptr) \
+({ \
+ typeof(*ptr) ___p1; \
+ __asm__ __volatile__ ("lr." pfx " %[p], %[c]\n" \
+ : [p]"=&r" (___p1), [c]"+A"(*ptr)); \
+ ___p1; \
+})
+
+#define ___smp_load_reserved32(ptr) \
+ ___smp_load_reservedN("w", ptr)
+
+#define ___smp_load_reserved64(ptr) \
+ ___smp_load_reservedN("d", ptr)
+
+#define __smp_load_reserved_relaxed(ptr) \
+({ \
+ typeof(*ptr) ___p1; \
+ compiletime_assert_atomic_lrsc_type(*ptr); \
+ if (sizeof(*ptr) == 4) { \
+ ___p1 = ___smp_load_reserved32(ptr); \
+ } else { \
+ ___p1 = ___smp_load_reserved64(ptr); \
+ } \
+ ___p1; \
+})
+
+#define __smp_load_reserved_acquire(ptr) \
+({ \
+ typeof(*ptr) ___p1; \
+ compiletime_assert_atomic_lrsc_type(*ptr); \
+ if (sizeof(*ptr) == 4) { \
+ ___p1 = ___smp_load_reserved32(ptr); \
+ } else { \
+ ___p1 = ___smp_load_reserved64(ptr); \
+ } \
+ 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
@@ -69,6 +129,38 @@ do { \
*/
#define smp_mb__after_spinlock() RISCV_FENCE(iorw,iorw)

+#define smp_cond_load_relaxed(ptr, cond_expr) \
+({ \
+ typeof(ptr) __PTR = (ptr); \
+ __unqual_scalar_typeof(*ptr) VAL; \
+ VAL = READ_ONCE(*__PTR); \
+ if (!cond_expr) { \
+ for (;;) { \
+ VAL = __smp_load_reserved_relaxed(__PTR); \
+ if (cond_expr) \
+ break; \
+ ALT_WRS_STO(); \
+ } \
+ } \
+ (typeof(*ptr))VAL; \
+})
+
+#define smp_cond_load_acquire(ptr, cond_expr) \
+({ \
+ typeof(ptr) __PTR = (ptr); \
+ __unqual_scalar_typeof(*ptr) VAL; \
+ VAL = smp_load_acquire(__PTR); \
+ if (!cond_expr) { \
+ for (;;) { \
+ VAL = __smp_load_reserved_acquire(__PTR); \
+ if (cond_expr) \
+ break; \
+ ALT_WRS_STO(); \
+ } \
+ } \
+ (typeof(*ptr))VAL; \
+})
+
#include <asm-generic/barrier.h>

#endif /* __ASSEMBLY__ */
diff --git a/arch/riscv/include/asm/errata_list.h b/arch/riscv/include/asm/errata_list.h
index 9e2888dbb5b1..e15af9986b1a 100644
--- a/arch/riscv/include/asm/errata_list.h
+++ b/arch/riscv/include/asm/errata_list.h
@@ -19,8 +19,9 @@
#define ERRATA_THEAD_NUMBER 1
#endif

-#define CPUFEATURE_SVPBMT 0
-#define CPUFEATURE_NUMBER 1
+#define CPUFEATURE_ZAWRS 0
+#define CPUFEATURE_SVPBMT 1
+#define CPUFEATURE_NUMBER 2

#ifdef __ASSEMBLY__

@@ -42,6 +43,20 @@ asm(ALTERNATIVE("sfence.vma %0", "sfence.vma", SIFIVE_VENDOR_ID, \
ERRATA_SIFIVE_CIP_1200, CONFIG_ERRATA_SIFIVE_CIP_1200) \
: : "r" (addr) : "memory")

+#define ZAWRS_WRS_NTO ".long 0x00d00073"
+#define ALT_WRS_NTO() \
+asm volatile(ALTERNATIVE( \
+ "nop\n\t", \
+ ZAWRS_WRS_NTO "\n\t", \
+ 0, CPUFEATURE_ZAWRS, CONFIG_RISCV_ISA_ZAWRS))
+
+#define ZAWRS_WRS_STO ".long 0x01d00073"
+#define ALT_WRS_STO() \
+asm volatile(ALTERNATIVE( \
+ "nop\n\t", \
+ ZAWRS_WRS_STO "\n\t", \
+ 0, CPUFEATURE_ZAWRS, CONFIG_RISCV_ISA_ZAWRS))
+
/*
* _val is marked as "will be overwritten", so need to set it to 0
* in the default case.
diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
index 4e2486881840..c7dd8cc38bec 100644
--- a/arch/riscv/include/asm/hwcap.h
+++ b/arch/riscv/include/asm/hwcap.h
@@ -51,7 +51,8 @@ extern unsigned long elf_hwcap;
* available logical extension id.
*/
enum riscv_isa_ext_id {
- RISCV_ISA_EXT_SSCOFPMF = RISCV_ISA_EXT_BASE,
+ RISCV_ISA_EXT_ZAWRS = RISCV_ISA_EXT_BASE,
+ RISCV_ISA_EXT_SSCOFPMF,
RISCV_ISA_EXT_SVPBMT,
RISCV_ISA_EXT_ID_MAX = RISCV_ISA_EXT_MAX,
};
diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
index fba9e9f46a8c..6c3a10ff5358 100644
--- a/arch/riscv/kernel/cpu.c
+++ b/arch/riscv/kernel/cpu.c
@@ -87,6 +87,7 @@ int riscv_of_parent_hartid(struct device_node *node)
* extensions by an underscore.
*/
static struct riscv_isa_ext_data isa_ext_arr[] = {
+ __RISCV_ISA_EXT_DATA(zawrs, RISCV_ISA_EXT_ZAWRS),
__RISCV_ISA_EXT_DATA(sscofpmf, RISCV_ISA_EXT_SSCOFPMF),
__RISCV_ISA_EXT_DATA(svpbmt, RISCV_ISA_EXT_SVPBMT),
__RISCV_ISA_EXT_DATA("", RISCV_ISA_EXT_MAX),
diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 12b05ce164bb..ce610d8a0e8d 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -199,6 +199,7 @@ void __init riscv_fill_hwcap(void)
} else {
SET_ISA_EXT_MAP("sscofpmf", RISCV_ISA_EXT_SSCOFPMF);
SET_ISA_EXT_MAP("svpbmt", RISCV_ISA_EXT_SVPBMT);
+ SET_ISA_EXT_MAP("zawrs", RISCV_ISA_EXT_ZAWRS);
}
#undef SET_ISA_EXT_MAP
}
@@ -250,6 +251,14 @@ struct cpufeature_info {
bool (*check_func)(unsigned int stage);
};

+static bool __init_or_module cpufeature_zawrs_check_func(unsigned int stage)
+{
+ if (stage == RISCV_ALTERNATIVES_EARLY_BOOT)
+ return false;
+
+ return riscv_isa_extension_available(NULL, ZAWRS);
+}
+
static bool __init_or_module cpufeature_svpbmt_check_func(unsigned int stage)
{
#ifdef CONFIG_RISCV_ISA_SVPBMT
@@ -266,6 +275,10 @@ static bool __init_or_module cpufeature_svpbmt_check_func(unsigned int stage)

static const struct cpufeature_info __initdata_or_module
cpufeature_list[CPUFEATURE_NUMBER] = {
+ {
+ .name = "zawrs",
+ .check_func = cpufeature_zawrs_check_func
+ },
{
.name = "svpbmt",
.check_func = cpufeature_svpbmt_check_func
--
2.35.3
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