linux-kernel - Re: [PATCH v7 5/6] arm64: mm: split linear mapping if BBML2 unsupported on secondary CPUs

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Message-ID: <1c9093da-3ae5-4d13-a56d-592f92a63d51@os.amperecomputing.com>
Date: Thu, 4 Sep 2025 10:54:10 -0700
From: Yang Shi <yang@...amperecomputing.com>
To: Catalin Marinas <catalin.marinas@....com>,
 Ryan Roberts <ryan.roberts@....com>
Cc: Will Deacon <will@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Ard Biesheuvel <ardb@...nel.org>, Dev Jain <dev.jain@....com>,
 scott@...amperecomputing.com, cl@...two.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH v7 5/6] arm64: mm: split linear mapping if BBML2
 unsupported on secondary CPUs



On 9/4/25 9:59 AM, Catalin Marinas wrote:
> On Fri, Aug 29, 2025 at 12:52:46PM +0100, Ryan Roberts wrote:
>> The kernel linear mapping is painted in very early stage of system boot.
>> The cpufeature has not been finalized yet at this point. So the linear
>> mapping is determined by the capability of boot CPU only. If the boot
>> CPU supports BBML2, large block mappings will be used for linear
>> mapping.
>>
>> But the secondary CPUs may not support BBML2, so repaint the linear
>> mapping if large block mapping is used and the secondary CPUs don't
>> support BBML2 once cpufeature is finalized on all CPUs.
>>
>> If the boot CPU doesn't support BBML2 or the secondary CPUs have the
>> same BBML2 capability with the boot CPU, repainting the linear mapping
>> is not needed.
>>
>> Repainting is implemented by the boot CPU, which we know supports BBML2,
>> so it is safe for the live mapping size to change for this CPU. The
>> linear map region is walked using the pagewalk API and any discovered
>> large leaf mappings are split to pte mappings using the existing helper
>> functions. Since the repainting is performed inside of a stop_machine(),
>> we must use GFP_ATOMIC to allocate the extra intermediate pgtables. But
>> since we are still early in boot, it is expected that there is plenty of
>> memory available so we will never need to sleep for reclaim, and so
>> GFP_ATOMIC is acceptable here.
>>
>> The secondary CPUs are all put into a waiting area with the idmap in
>> TTBR0 and reserved map in TTBR1 while this is performed since they
>> cannot be allowed to observe any size changes on the live mappings. Some
>> of this infrastructure is reused from the kpti case. Specifically we
>> share the same flag (was __idmap_kpti_flag, now idmap_kpti_bbml2_flag)
>> since it means we don't have to reserve any extra pgtable memory to
>> idmap the extra flag.
>>
>> Co-developed-by: Yang Shi <yang@...amperecomputing.com>
>> Signed-off-by: Yang Shi <yang@...amperecomputing.com>
>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> I think this works, so:
>
> Reviewed-by: Catalin Marinas <catalin.marinas@....com>

Thank you!

>
> However, I wonder how likely we are to find this combination in the
> field to be worth carrying this code upstream. With kpti, we were aware
> of platforms requiring it but is this also the case for BBM? If not, I'd
> keep the patch out until we get a concrete example.

At least we (Ampere) are very very unlikely to ship asymmetric systems 
AFAICT.

Thanks,
Yang

>