[RFC PATCH] riscv: Add Zawrs support for spinlocks
Heiko Stübner
heiko at sntech.de
Tue Jun 14 09:29:20 PDT 2022
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 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
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 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
> >
> >
> > _______________________________________________
> > linux-riscv mailing list
> > linux-riscv at lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-riscv
>
>
>
>
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