[RFC PATCH] riscv: Add Zawrs support for spinlocks
Atish Patra
atishp at atishpatra.org
Tue Jun 14 08:34:48 PDT 2022
On Thu, Jun 2, 2022 at 7:11 AM Christoph Muellner
<christoph.muellner at vrull.eu> wrote:
>
> 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 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.
> A similar patch could add support for the PAUSE instruction of
> the Zihintpause ISA extension.
>
FYI..The initial Zihintpause support patch
https://www.spinics.net/lists/kernel/msg4380326.html
> 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.
>
> Signed-off-by: Christoph Muellner <christoph.muellner at vrull.io>
> ---
> arch/riscv/Kconfig | 10 +++
> arch/riscv/include/asm/barrier.h | 97 ++++++++++++++++++++++++++++
> arch/riscv/include/asm/errata_list.h | 12 +++-
> arch/riscv/include/asm/hwcap.h | 3 +-
> arch/riscv/kernel/cpu.c | 1 +
> arch/riscv/kernel/cpufeature.c | 13 ++++
> 6 files changed, 133 insertions(+), 3 deletions(-)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 905e550e0fd3..054872317d4a 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -358,6 +358,16 @@ config RISCV_ISA_C
>
> If you don't know what to do here, say Y.
>
> +config RISCV_ISA_ZAWRS
> + bool "Zawrs extension support"
> + 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..69b8f1f4b80c 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 +44,69 @@ do { \
> ___p1; \
> })
>
> +#if __riscv_xlen == 64
> +
> +#define __riscv_lrsc_word(t) \
> + (sizeof(t) == sizeof(int) || \
> + sizeof(t) == sizeof(long))
> +
> +#define __riscv_lr(ptr) \
> + sizeof(*ptr) == sizeof(int) ? "lr.w" : "lr.d"
> +
> +#elif __riscv_xlen == 32
> +
> +#define __riscv_lrsc_word(ptr) \
> + (sizeof(*ptr) == sizeof(int))
> +
> +#define __riscv_lr(t) "lr.w"
> +
> +#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) == 32) { \
> + ___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) == 32) { \
> + ___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 +134,38 @@ 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(); \
> + } \
> + } \
> + (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(); \
> + } \
> + } \
> + (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..b9aa0b346493 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 +43,13 @@ 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 ".long 0x1000073"
> +#define ALT_WRS() \
> +asm volatile(ALTERNATIVE( \
> + "nop\n\t", \
> + ZAWRS_WRS "\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 dea3ea19deee..fc2c47a1784b 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
> }
> @@ -251,6 +252,14 @@ 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
> @@ -267,6 +276,10 @@ 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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--
Regards,
Atish
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