Re: [RFC PATCH v2 0/3] Set DBM bit on writable pages

[Leonardo Bras]

On Tue, Oct 27, 2026 at 11:44:15AM +0000, Leonardo Bras wrote:
> On Mon, Oct 26, 2026 at 12:08:59PM -0700, Oliver Upton wrote:
> > On Mon, Oct 26, 2026 at 02:44:06PM +0000, Leonardo Bras wrote:
> > > When HAFDBS is enabled (VTCR_EL2.{HA,HD}={1,1}), having a DBM bit allows 
> > > hardware to promote a non-writable page to writable by setting (S2AP[1]=1),
> > > without having to take a fault.
> > > 
> > > The reason for this patchset is to add the DBM bit when setting-up writable 
> > > pages, so the code is prepared for future patches that make use of 
> > > HDBSS/HACDBS to accelerate dirty-tracking. Write-bit detection still 
> > > happens based only in S2AP[1], so no change in that aspect.
> > > 
> > > Having DBM set on block entries is undesired, as we benefit from the first 
> > > fault for splitting it in PAGE_SIZE entries, which is the granularity used 
> > > to track the dirty pages in KVM. So introduce a new walker that, on top of 
> > > clearing S2AP[1] for every entry, also clear DBM for block entries.
> > > 
> > > Also introduce a new VCPU request to reload Stage2, as it will be needed on 
> > > future HDBSS/HAFDBS patchsets. 
> > > 
> > > Please let me know of what do you think!
> > 
> > I'm pretty sure I shared this feedback before, but patches that enable
> > DBM do not make sense in isolation. We need a single series that
> > contains the DBM enablement as well as the corresponding HDBSS support.
> >
> 
> Right, 
> My reasoning is that we can discuss this new version, and make the needed 
> changes, so Tian could pick this for his own patch series for the next 
> version. I do not intend to get this merged by itself.
> 
> Next time I will make sure to state this in the cover letter so it's clear.
>  
> > > Changes since RFCv1:
> > > - Dropped patch #5 on enabling HAFDBS on non-dirty-tracking guests, as it 
> > >   proved to be counter-productive on wrprotect, contrary to expectations
> > > - No more moving writable-bit to DBM. That does not seem to be the intended 
> > >   use. It makes sense and it's easier to convert code to new DBM approach.
> > 
> > Sorry, where was this concluded?
> > 
> 
> Sorry, this is my viewpoint after re-reading the documentation and 
> being really pedantic while trying to get a proper answer from the people 
> responsible for keeping/improving it. 
> 
> > The abstract permission flags need to have a WRITE an DIRTY state, which
> > we then map to hardware based on the capabilities of the system.
> > 
> > !FEAT_HAFDBS:
> > 
> > 	DIRTY	=> S2AP[1]
> > 	WRITE	=> S2AP[1]
> > 
> > FEAT_HAFDBS:
> > 
> > 	DIRTY	=> S2AP[1]
> > 	WRITE	=> DBM
> > 
> 
> I agree that it could work, and my first version was based on that.
> 
> But looks weird to me that we have to decide the writable bit based on 
> having a feature enabled.
> 
> Take in account that we can only have DBM being the writable bit while 
> HAFDBS is enabled (not only present), and that it is just enabled during 
> dirty-tracking. 
> 


> It means we would need to convert the whole pagetable between DIRTY=S2AP[1] 
> and DIRTY=DBM every time we enable or disable dirty-tracking.

s/DIRTY/WRITE/


> 
> My suggestion with this series is to have WRITE=DIRTY=S2AP[1] at all 
> times, and have DBM to act as a "can be promoted to writable on write" bit.
> This way we don't actually need to convert between encodings, and it still 
> works and complies with the architecture.
> 
> > Beyond making the whole HAFDBS easy to reason about, I want this for
> > FEAT_S2PIE which reimagines bit 7 as a literal dirty bit that can be
> > managed by software or hardware.
> 
> Humm, and would not that work with the above suggested approach?
> I mean, bit 7 (Dirty, S2AP[1]) will both mean WRITE/DIRTY, but it will be 
> able to get promoted (nonW->W, or clean->dirty) based on the indirect 
> permissions.
> 
> What do you think?
> 
> Thanks for reviewing,
> Leo