Re: [RFC PATCH 0/3] arm64: support FEAT_BBM level 2 and large block mapping when rodata=full

[Yang Shi <yang@os.amperecomputing.com>]

On 12/10/24 3:31 AM, Will Deacon wrote:
> On Mon, Nov 18, 2024 at 10:16:07AM -0800, Yang Shi wrote:
>> When rodata=full kernel linear mapping is mapped by PTE due to arm's
>> break-before-make rule.
>>
>> This resulted in a couple of problems:
>>    - performance degradation
>>    - more TLB pressure
>>    - memory waste for kernel page table
>>
>> There are some workarounds to mitigate the problems, for example, using
>> rodata=on, but this compromises the security measurement.
>>
>> With FEAT_BBM level 2 support, splitting large block page table to
>> smaller ones doesn't need to make the page table entry invalid anymore.
>> This allows kernel split large block mapping on the fly.
> I think you can still get TLB conflict aborts in this case, so this
> doesn't work. Hopefully the architecture can strengthen this in the
> future to give you what you need.

Hi Will,

Thanks for responding. This is a little bit surprising. I thought 
FEAT_BBM level 2 can handle the TLB conflict gracefully. At least its 
description made me assume so. And Catalin also mentioned FEAT_BBM level 
2 can be used to split vmemmap page table in HVO patch discussion 
(https://lore.kernel.org/all/Zo68DP6siXfb6ZBR@arm.com/).

It sounds a little bit contradicting if the TLB conflict still can 
happen with FEAT_BBM level 2. It makes the benefit of FEAT_BBM level 2 
much less than expected.

Is it out of question to handle the TLB conflict aborts? IIUC we should 
just need flush TLB then resume, and it doesn't require to hold any 
locks as well.

And I chatted with our architects, I was told the TLB conflict abort 
doesn't happen on AmpereOne. Maybe this is why I didn't see the problem 
when I tested the patches.

Thanks,
Yang


>
> Will