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: <ceaf604f-8ad7-4e19-812e-7eeace10d835@arm.com>
Date: Fri, 19 Sep 2025 12:49:22 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Will Deacon <will@...nel.org>
Cc: 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>, 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 19/09/2025 12:27, Will Deacon wrote:
> On Fri, Sep 19, 2025 at 11:08:47AM +0100, Ryan Roberts wrote:
>> 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 surprised if these SoCs are booting on the X4 but who knows.

Ahh. You can probably tell I'm a bit naive to some of this system level stuff...
I had assumed they would want to boot on the big CPU to reduce boot time.

> 
> Lemme have another look at applying the patch with fresh eyes, but I do
> wonder whether having X4 on the allow list really makes any sense. Are
> there any SoCs out there that _don't_ pair it with CPUs that aren't on
> the allow list? (apologies for the double negative).

Hmm, that's a fair question. I'm not aware of any. So I guess the simplest
solution is to remove X4 from the allow list and ditch fourth patch.


> 
> Will