linux-kernel - Re: [PATCH] Documentation/features: mark BATCHED_UNMAP_TLB

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Message-ID: <09dd1026-9e3f-b9be-b5a5-82771642348d@arm.com>
Date:   Tue, 23 Feb 2021 11:40:09 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Barry Song <song.bao.hua@...ilicon.com>, corbet@....net,
        linux-doc@...r.kernel.org, akpm@...ux-foundation.org,
        linux-mm@...ck.org
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linuxarm@...neuler.org, Mel Gorman <mgorman@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH] Documentation/features: mark BATCHED_UNMAP_TLB_FLUSH
 doesn't apply to ARM64



On 2/23/21 6:02 AM, Barry Song wrote:
> BATCHED_UNMAP_TLB_FLUSH is used on x86 to do batched tlb shootdown by
> sending one IPI to TLB flush all entries after unmapping pages rather
> than sending an IPI to flush each individual entry.
> On arm64, tlb shootdown is done by hardware. Flush instructions are
> innershareable. The local flushes are limited to the boot (1 per CPU)
> and when a task is getting a new ASID.

Is there any previous discussion around this ?

> So marking this feature as "TODO" is not proper. ".." isn't good as
> well. So this patch adds a "N/A" for this kind of features which are
> not needed on some architectures.
> 
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will@...nel.org>
> Signed-off-by: Barry Song <song.bao.hua@...ilicon.com>
> ---
>  Documentation/features/arch-support.txt        | 1 +
>  Documentation/features/vm/TLB/arch-support.txt | 2 +-
>  2 files changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/features/arch-support.txt b/Documentation/features/arch-support.txt
> index d22a1095e661..118ae031840b 100644
> --- a/Documentation/features/arch-support.txt
> +++ b/Documentation/features/arch-support.txt
> @@ -8,4 +8,5 @@ The meaning of entries in the tables is:
>      | ok |  # feature supported by the architecture
>      |TODO|  # feature not yet supported by the architecture
>      | .. |  # feature cannot be supported by the hardware
> +    | N/A|  # feature doesn't apply to the architecture

NA might be better here. s/doesn't apply/not applicable/ in order to match NA.
Still wondering if NA is really needed when there is already ".." ? Regardless
either way should be fine.

>  
> diff --git a/Documentation/features/vm/TLB/arch-support.txt b/Documentation/features/vm/TLB/arch-support.txt
> index 30f75a79ce01..0d070f9f98d8 100644
> --- a/Documentation/features/vm/TLB/arch-support.txt
> +++ b/Documentation/features/vm/TLB/arch-support.txt
> @@ -9,7 +9,7 @@
>      |       alpha: | TODO |
>      |         arc: | TODO |
>      |         arm: | TODO |
> -    |       arm64: | TODO |
> +    |       arm64: | N/A  |
>      |         c6x: |  ..  |
>      |        csky: | TODO |
>      |       h8300: |  ..  |
>