Messages in this thread | | | From | Heiko Stübner <> | Subject | Re: [RFC PATCH] riscv: Add Zawrs support for spinlocks | Date | Tue, 14 Jun 2022 18:29:20 +0200 |
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Am Dienstag, 14. Juni 2022, 17:34:48 CEST schrieb Atish Patra: > On Thu, Jun 2, 2022 at 7:11 AM Christoph Muellner > <christoph.muellner@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
Out of curiosity, is this functionally equivalent to WRS, So can you just replace WRS with Pause?
If so, ALTERNATIVE_2 would make that pretty easy.
Are there performance differences between them?
Heiko
> > > 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@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 > > > > > > _______________________________________________ > > linux-riscv mailing list > > linux-riscv@lists.infradead.org > > http://lists.infradead.org/mailman/listinfo/linux-riscv > > > >
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