Re: [PATCH v5 01/15] KVM: arm64: pgtables: Change write bit from S2AP_W to DBM

[Leonardo Bras <leo.bras@arm.com>]

On Wed, Sep 30, 2026 at 10:44:26AM +0800, Tian Zheng wrote:
> On 9/30/2026 8:25 AM, Oliver Upton wrote:
> > Hmm... I'd actually like to see this structured where the "write" bit
> > could either be S2AP_W or DBM depending on if the system supports
> > FEAT_HAFDBS. Look at how HVHE is handled for selecting the right AP
> > encoding for the hyp stage-1 page tables.
> > 
> 
> Hi Oliver,
> 
> Thanks for the suggestion. I missed the case where FEAT_HAFDBS is not
> supported. Systems without the feature should keep the original
> encoding, with S2AP[1] as the write permission bit. I'll follow the HVHE
> pattern to select between the two encodings in the next version.
> 
> Thanks!
> Tian
> 

Hi Oliver, Tian,

After some digging, I think it would be better if we kept using S2AP[1] as 
a writable bit, while DBM can be treated as "Can promote a non-writable 
page to writable if HAFDBS=on". 

I know it's not that intuitive, as the Arm ARM even introduced the concept 
of Writable-clean an Writable-dirty, but I think it will be the best way of 
referring to what was intended of the bit.

I wrote down what I mean by that:
https://lore.kernel.org/all/20261026144413.441566-1-leo.bras@arm.com/

tl;dr: 
- DBM + S2AP[1] is set on writable entries (KVM_PGTABLE_PROT_W)
- Writable detection is made only on S2AP[1], so no changes
- Wrprotect sets S2AP[1]=0 from the PTE, and also DBM=0 if it's a block

On dirty-tracking:
- If HAFDBS=0 : no functional change, DBM bit there but unused
- If HDBSS=1
  - Pages: writable-clean promoted to writable-dirty on write, HDBSS does 
    the tracking
  - Blocks: faults on write, splits in pages, pages get DBM=1. 
  	(argh, I think we need a PROT_DBM for that) 

Does that make sense?

Thanks!
Leo