linux-kernel - Re: [PATCH] LoongArch: Support dbar with different hints

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Date:   Tue, 16 May 2023 21:19:56 +0800
From:   Xi Ruoyao <xry111@...111.site>
To:     Huacai Chen <chenhuacai@...ngson.cn>,
        Huacai Chen <chenhuacai@...nel.org>
Cc:     loongarch@...ts.linux.dev, linux-arch@...r.kernel.org,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Guo Ren <guoren@...nel.org>, Xuerui Wang <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        linux-kernel@...r.kernel.org, loongson-kernel@...ts.loongnix.cn,
        Jun Yi <yijun@...ngson.cn>
Subject: Re: [PATCH] LoongArch: Support dbar with different hints

On Tue, 2023-05-16 at 20:45 +0800, Huacai Chen wrote:
> Traditionally, LoongArch uses "dbar 0" (full completion barrier) for
> everything. But the full completion barrier is a performance killer, so
> Loongson-3A6000 and newer processors introduce different hints:
> 
> Bit4: ordering or completion (0: completion, 1: ordering)
> Bit3: barrier for previous read (0: true, 1: false)
> Bit2: barrier for previous write (0: true, 1: false)
> Bit1: barrier for succeeding read (0: true, 1: false)
> Bit0: barrier for succedding write (0: true, 1: false)
> 
> Hint 0x700: barrier for "read after read" from the same address, which
> is needed by LL-SC loops.

Great!  I guess Xuerui would add this into his weekly news :).

I don't really understand these C++-memory-model-like concepts so I'll
not review the "orwrw" parts, but...

/* snip */

> diff --git a/arch/loongarch/mm/tlbex.S b/arch/loongarch/mm/tlbex.S
> index 244e2f5aeee5..240ced55586e 100644
> --- a/arch/loongarch/mm/tlbex.S
> +++ b/arch/loongarch/mm/tlbex.S
> @@ -184,7 +184,7 @@ tlb_huge_update_load:
>         ertn
>  
>  nopage_tlb_load:
> -       dbar            0
> +       dbar            0x700

There is no LL/SC loop here.  I guess this shares a same internal uarch
logic as the LL/SC loop, but there should be a clarification for 0x700
in the commit message.

>         csrrd           ra, EXCEPTION_KS2
>         la_abs          t0, tlb_do_page_fault_0
>         jr              t0
> @@ -333,7 +333,7 @@ tlb_huge_update_store:
>         ertn
>  
>  nopage_tlb_store:
> -       dbar            0
> +       dbar            0x700
>         csrrd           ra, EXCEPTION_KS2
>         la_abs          t0, tlb_do_page_fault_1
>         jr              t0
> @@ -480,7 +480,7 @@ tlb_huge_update_modify:
>         ertn
>  
>  nopage_tlb_modify:
> -       dbar            0
> +       dbar            0x700
>         csrrd           ra, EXCEPTION_KS2
>         la_abs          t0, tlb_do_page_fault_1
>         jr              t0

-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University