Re: [RFC 5/6] RISC-V: Move spinwait booting method to its own config

[Atish Patra <atishp@atishpatra.org>]

On Mon, Dec 13, 2021 at 5:02 AM Anup Patel <anup@brainfault.org> wrote:
>
> On Sat, Dec 4, 2021 at 5:51 AM Atish Patra <atishp@atishpatra.org> wrote:
> >
> > From: Atish Patra <atishp@rivosinc.com>
> >
> > The spinwait booting method should only be used for platforms with older
> > firmware without SBI HSM extension or M-mode firmware because spinwait
> > method can't support cpu hotplug, kexec or sparse hartid. It is better
> > to move the entire spinwait implementation to its own config which can
> > be disabled if required. It is enabled by default to maintain backward
> > compatibility and M-mode Linux.
> >
> > Signed-off-by: Atish Patra <atishp@rivosinc.com>
> > ---
> >  arch/riscv/Kconfig          | 14 ++++++++++++++
> >  arch/riscv/kernel/Makefile  |  3 ++-
> >  arch/riscv/kernel/cpu_ops.c |  8 ++++++++
> >  arch/riscv/kernel/head.S    |  6 +++---
> >  arch/riscv/kernel/head.h    |  2 ++
> >  5 files changed, 29 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index 821252b65f89..4afb42d5707d 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -403,6 +403,20 @@ config RISCV_SBI_V01
> >           This config allows kernel to use SBI v0.1 APIs. This will be
> >           deprecated in future once legacy M-mode software are no longer in use.
> >
> > +config RISCV_BOOT_SPINWAIT
> > +       bool "Spinwait booting method"
> > +       depends on SMP
> > +       default y
> > +       help
> > +         This enables support for booting Linux via spinwait method. In the
> > +         spinwait method, all cores randomly jump to Linux. One of the core
> > +         gets chosen via lottery and all other keeps spinning on a percpu
> > +         variable. This method can not support cpu hotplug and sparse hartid
> > +         scheme. It should be only enabled for M-mode Linux or platforms relying
> > +         on older firmware without SBI HSM extension. All other platform should
> > +         rely on ordered booing via SBI HSM extension which gets chosen
> > +          dynamically at runtime if the firmware supports it.
> > +
> >  config KEXEC
> >         bool "Kexec system call"
> >         select KEXEC_CORE
> > diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> > index 3397ddac1a30..612556faa527 100644
> > --- a/arch/riscv/kernel/Makefile
> > +++ b/arch/riscv/kernel/Makefile
> > @@ -43,7 +43,8 @@ obj-$(CONFIG_FPU)             += fpu.o
> >  obj-$(CONFIG_SMP)              += smpboot.o
> >  obj-$(CONFIG_SMP)              += smp.o
> >  obj-$(CONFIG_SMP)              += cpu_ops.o
> > -obj-$(CONFIG_SMP)              += cpu_ops_spinwait.o
> > +
> > +obj-$(CONFIG_RISCV_BOOT_SPINWAIT) += cpu_ops_spinwait.o
> >  obj-$(CONFIG_MODULES)          += module.o
> >  obj-$(CONFIG_MODULE_SECTIONS)  += module-sections.o
> >
> > diff --git a/arch/riscv/kernel/cpu_ops.c b/arch/riscv/kernel/cpu_ops.c
> > index c1e30f403c3b..170d07e57721 100644
> > --- a/arch/riscv/kernel/cpu_ops.c
> > +++ b/arch/riscv/kernel/cpu_ops.c
> > @@ -15,7 +15,15 @@
> >  const struct cpu_operations *cpu_ops[NR_CPUS] __ro_after_init;
> >
> >  extern const struct cpu_operations cpu_ops_sbi;
> > +#ifdef CONFIG_RISCV_BOOT_SPINWAIT
> >  extern const struct cpu_operations cpu_ops_spinwait;
> > +#else
> > +const struct cpu_operations cpu_ops_spinwait = {
> > +       .name           = "",
> > +       .cpu_prepare    = NULL,
> > +       .cpu_start      = NULL,
> > +};
> > +#endif
> >
> >  void __init cpu_set_ops(int cpuid)
> >  {
> > diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> > index 9f16bfe9307e..4a694e15b95b 100644
> > --- a/arch/riscv/kernel/head.S
> > +++ b/arch/riscv/kernel/head.S
> > @@ -259,7 +259,7 @@ pmp_done:
> >         li t0, SR_FS
> >         csrc CSR_STATUS, t0
> >
> > -#ifdef CONFIG_SMP
> > +#ifdef CONFIG_RISCV_BOOT_SPINWAIT
> >         li t0, CONFIG_NR_CPUS
> >         blt a0, t0, .Lgood_cores
> >         tail .Lsecondary_park
> > @@ -285,7 +285,7 @@ pmp_done:
> >         beq t0, t1, .Lsecondary_start
> >
> >  #endif /* CONFIG_XIP */
> > -#endif /* CONFIG_SMP */
> > +#endif /* CONFIG_RISCV_BOOT_SPINWAIT */
> >
> >  #ifdef CONFIG_XIP_KERNEL
> >         la sp, _end + THREAD_SIZE
> > @@ -344,7 +344,7 @@ clear_bss_done:
> >         call soc_early_init
> >         tail start_kernel
> >
> > -#ifdef CONFIG_SMP
> > +#if defined(CONFIG_SMP) && defined(CONFIG_RISCV_BOOT_SPINWAIT)
>
> The RISCV_BOOT_SPINWAIT option already depends on SMP.
>
> Do you still need to check defined(CONFIG_SMP) here ?
>

Nope. I guess this one slipped through the cracks. All other related
#ifdef have only CONFIG_RISCV_BOOT_SPINWAIT
I will fix it in v2.

> Regards,
> Anup
>
> >  .Lsecondary_start:
> >         /* Set trap vector to spin forever to help debug */
> >         la a3, .Lsecondary_park
> > diff --git a/arch/riscv/kernel/head.h b/arch/riscv/kernel/head.h
> > index 5393cca77790..726731ada534 100644
> > --- a/arch/riscv/kernel/head.h
> > +++ b/arch/riscv/kernel/head.h
> > @@ -16,7 +16,9 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa);
> >  asmlinkage void __init __copy_data(void);
> >  #endif
> >
> > +#ifdef CONFIG_RISCV_BOOT_SPINWAIT
> >  extern void *__cpu_spinwait_stack_pointer[];
> >  extern void *__cpu_spinwait_task_pointer[];
> > +#endif
> >
> >  #endif /* __ASM_HEAD_H */
> > --
> > 2.33.1
> >



-- 
Regards,
Atish