Re: [PATCH v3 1/4] Revert "Documentation/features: mark BATCHED_UNMAP_TLB_FLUSH doesn't apply to ARM64"

From: Anshuman Khandual <anshuman.khandual@arm.com>
To: Yicong Yang <yangyicong@huawei.com>,
	akpm@linux-foundation.org, linux-mm@kvack.org,
	linux-arm-kernel@lists.infradead.org, x86@kernel.org,
	catalin.marinas@arm.com, will@kernel.org,
	linux-doc@vger.kernel.org
Cc: corbet@lwn.net, peterz@infradead.org, arnd@arndb.de,
	linux-kernel@vger.kernel.org, darren@os.amperecomputing.com,
	yangyicong@hisilicon.com, huzhanyuan@oppo.com,
	lipeifeng@oppo.com, zhangshiming@oppo.com, guojian@oppo.com,
	realmz6@gmail.com, linux-mips@vger.kernel.org,
	openrisc@lists.librecores.org, linuxppc-dev@lists.ozlabs.org,
	linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org,
	Barry Song <21cnbao@gmail.com>,
	wangkefeng.wang@huawei.com, xhao@linux.alibaba.com,
	prime.zeng@hisilicon.com, Barry Song <v-songbaohua@oppo.com>
Subject: Re: [PATCH v3 1/4] Revert "Documentation/features: mark BATCHED_UNMAP_TLB_FLUSH doesn't apply to ARM64"
Date: Fri, 9 Sep 2022 09:56:21 +0530	[thread overview]
Message-ID: <cc68c8c0-0db1-1678-2095-54383c1383ad@arm.com> (raw)
In-Reply-To: <20220822082120.8347-2-yangyicong@huawei.com>

On 8/22/22 13:51, Yicong Yang wrote:
> From: Barry Song <v-songbaohua@oppo.com>
> 
> This reverts commit 6bfef171d0d74cb050112e0e49feb20bfddf7f42.
> 
> I was wrong. Though ARM64 has hardware TLB flush, but it is not free
> and it is still expensive.
> We still have a good chance to enable batched and deferred TLB flush
> on ARM64 for memory reclamation. A possible way is that we only queue
> tlbi instructions in hardware's queue. When we have to broadcast TLB,
> we broadcast it by dsb. We just need to get adapted the existing
> BATCHED_UNMAP_TLB_FLUSH.
> 
> Tested-by: Xin Hao <xhao@linux.alibaba.com>
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
> ---
>  Documentation/features/arch-support.txt        | 1 -
>  Documentation/features/vm/TLB/arch-support.txt | 2 +-
>  2 files changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/Documentation/features/arch-support.txt b/Documentation/features/arch-support.txt
> index 118ae031840b..d22a1095e661 100644
> --- a/Documentation/features/arch-support.txt
> +++ b/Documentation/features/arch-support.txt
> @@ -8,5 +8,4 @@ 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
>  
> diff --git a/Documentation/features/vm/TLB/arch-support.txt b/Documentation/features/vm/TLB/arch-support.txt
> index 039e4e91ada3..1c009312b9c1 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: | N/A  |
> +    |       arm64: | TODO |
>      |        csky: | TODO |
>      |     hexagon: | TODO |
>      |        ia64: | TODO |

I believe this patch is not needed, which explicitly reverts an
older commit. Instead when ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
framework gets enabled on arm64, the same patch could just drop
'NA' as possible values for arch support for a give feature in
file Documentation/features/arch-support.txt.