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SubjectRe: [PATCH 1/3] Loongson-2F: Flush the branch target history such as BTB and RAS
Wu Zhangjin wrote:
> From: Wu Zhangjin <wuzhangjin@gmail.com>
>
> As the Chapter 15: "Errata: Issue of Out-of-order in loongson"[1] shows, to
> workaround the Issue of Loongson-2F,We need to do:
>
> "When switching from user model to kernel model, you should flush the branch
> target history such as BTB and RAS."

Just wondered, model or mode?

> This patch did clear BTB(branch target buffer), forbid RAS(row address strobe)
> via Loongson-2F's 64bit diagnostic register.

Are you sure that RAS represents "Row Address Strobe", not "Return
Address Stack?"

By the way, we have a similar local workaround for vr55xx processors
when switching from kernel mode to user mode. It's not necessarily
related to out-of-order issues, but we need to prevent the processor
from doing instruction prefetch beyond "eret" instruction.

In the long term, it would be appreciated that the kernel has a set
of hooks when switching KUX-modes, so that each machine could have
his own, processor-specific treatmens.

Shinya

> [1] Chinese Version: http://www.loongson.cn/uploadfile/file/200808211
> [2] English Version of Chapter 15:
> http://groups.google.com.hk/group/loongson-dev/msg/e0d2e220958f10a6?dmode=source
>
> Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
> ---
> arch/mips/include/asm/stackframe.h | 19 +++++++++++++++++++
> 1 files changed, 19 insertions(+), 0 deletions(-)
>
> diff --git a/arch/mips/include/asm/stackframe.h b/arch/mips/include/asm/stackframe.h
> index 3b6da33..b84cfda 100644
> --- a/arch/mips/include/asm/stackframe.h
> +++ b/arch/mips/include/asm/stackframe.h
> @@ -121,6 +121,25 @@
> .endm
> #else
> .macro get_saved_sp /* Uniprocessor variation */
> +#ifdef CONFIG_CPU_LOONGSON2F
> + /*
> + * Clear BTB(branch target buffer), forbid RAS(row address
> + * strobe) to workaround the Out-of-oder Issue in Loongson2F
> + * via it's diagnostic register.
> + */
> + move k0, ra
> + jal 1f
> + nop
> +1: jal 1f
> + nop
> +1: jal 1f
> + nop
> +1: jal 1f
> + nop
> +1: move ra, k0
> + li k0, 3
> + mtc0 k0, $22
> +#endif /* CONFIG_CPU_LOONGSON2F */
> #if defined(CONFIG_32BIT) || defined(KBUILD_64BIT_SYM32)
> lui k1, %hi(kernelsp)
> #else
--
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