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

[Ryan Roberts <ryan.roberts@arm.com>]

On 04/09/2025 17:59, 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@os.amperecomputing.com>
>> Signed-off-by: Yang Shi <yang@os.amperecomputing.com>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> 
> I think this works, so:
> 
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

Thanks!

> 
> 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.

Cortex-X4 supports BBML2_NOABORT (and is in the allow list). According to
Wikipedia [1], X4 is in:

 - Google Tensor G4 [2]
 - MediaTek Dimensity 9300/9300+ [3]
 - Qualcomm Snapdragon 8 Gen 3 [4]

And in each of those SoCs, the X4s are paired with A720 and A520 cores.

To my knowledge, neither A720 nor A520 support BBML2_NOABORT. Certainly they are
not currently in the allow list. So on that basis, I think the require the
fallback path, assuming these platforms use one of the X4 cores as the boot CPU.

[1] https://en.wikipedia.org/wiki/ARM_Cortex-X4
[2] https://en.wikipedia.org/wiki/Google_Tensor
[3] https://en.wikipedia.org/wiki/List_of_MediaTek_systems_on_chips
[4] https://en.wikipedia.org/wiki/List_of_Qualcomm_Snapdragon_systems_on_chips

Thanks,
Ryan