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SubjectRe: [PATCH v1] RISC-V: Only default to spinwait on SBI-0.1 and M-mode
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On Thu, 21 Apr 2022 10:03:55 PDT (-0700), Palmer Dabbelt wrote:
> From: Palmer Dabbelt <palmer@rivosinc.com>
>
> The spinwait boot method has been superseeded by the SBI HSM extension
> for some time now, but it still enabled by default. This causes some
> issues on large hart count systems, which will hang if a physical hart
> exists that is larger than NR_CPUS.
>
> Users on modern SBI implemenation don't need spinwait, and while it's
> probably possible to deal with some of the spinwait issues let's just
> restrict the default to systems that are likely to actually use it.
>
> Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
> ---
> This seems to be the source of many of my new hangs when trying to test
> the NR_CPUS=512 support. It's not really related, just fallout from
> testing different setups.
> ---
> arch/riscv/Kconfig | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 00fd9c548f26..dd5e975abe37 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -359,7 +359,7 @@ config RISCV_SBI_V01
> config RISCV_BOOT_SPINWAIT
> bool "Spinwait booting method"
> depends on SMP
> - default y
> + default y if RISCV_SBI_V01 || RISCV_M_MODE
> help
> This enables support for booting Linux via spinwait method. In the
> spinwait method, all cores randomly jump to Linux. One of the cores
> @@ -370,6 +370,12 @@ config RISCV_BOOT_SPINWAIT
> rely on ordered booting via SBI HSM extension which gets chosen
> dynamically at runtime if the firmware supports it.
>
> + Since spinwait is incompatible with sparse hart IDs, it requires
> + NR_CPUS be large enough to contain the physical hart ID of the first
> + hart to enter Linux.
> +
> + If unsure what to do here, say N.
> +
> config KEXEC
> bool "Kexec system call"
> select KEXEC_CORE

This is on for-next, with some typos fixed.

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