linux-kernel - Re: [PATCH v8 0/5] arm64: support FEAT_BBM level 2 and large block mapping when rodata=full

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Message-ID: <fc43aa07-d4ad-47d5-8381-61d4a9b8c467@arm.com>
Date: Fri, 19 Sep 2025 11:08:47 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Will Deacon <will@...nel.org>, catalin.marinas@....com,
 akpm@...ux-foundation.org, david@...hat.com, lorenzo.stoakes@...cle.com,
 ardb@...nel.org, dev.jain@....com, scott@...amperecomputing.com,
 cl@...two.org, Yang Shi <yang@...amperecomputing.com>
Cc: kernel-team@...roid.com, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v8 0/5] arm64: support FEAT_BBM level 2 and large block
 mapping when rodata=full

On 18/09/2025 22:10, Will Deacon wrote:
> On Wed, 17 Sep 2025 12:02:06 -0700, Yang Shi wrote:
>> On systems with BBML2_NOABORT support, it causes the linear map to be mapped
>> with large blocks, even when rodata=full, and leads to some nice performance
>> improvements.
>>
>> Ryan tested v7 on an AmpereOne system (a VM with 12G RAM) in all 3 possible
>> modes by hacking the BBML2 feature detection code:
>>
>> [...]
> 
> Applied patches 1 and 3 to arm64 (for-next/mm), thanks!
> 
> [1/5] arm64: Enable permission change on arm64 kernel block mappings
>       https://git.kernel.org/arm64/c/a660194dd101
> [3/5] arm64: mm: support large block mapping when rodata=full
>       https://git.kernel.org/arm64/c/a166563e7ec3
> 
> I also picked up the BBML allow-list addition (second patch) on
> for-next/cpufeature.
> 
> The fourth patch ("arm64: mm: split linear mapping if BBML2 unsupported
> on secondary CPUs") has some really horrible conflicts. These are partly
> due to some of the type cleanups on for-next/mm but I think mainly due
> to Kevin's kpti rework that landed after -rc1.

Thanks Will, although I'm nervous that without this patch, some platforms might
not boot; Wikipedia tells me that there are some Google, Mediatek and Qualcomm
SoCs that pair X4 CPUs (which is on the BBML2_NOABORT allow list) with A720
and/or A520 (which are not). See previous mail at [1].

I'd be happy to rebase it if you can let me know the prefered base SHA/tree?

[1]
https://lore.kernel.org/linux-arm-kernel/11f84d00-8c76-402d-bbad-014a3542992f@arm.com/

Thanks,
Ryan


> 
> So I think the best bet might be to leave that one for next time, if
> that's ok?
> 
> Cheers,