Re: [PATCH 13/22] KVM: arm64: Set dirty state at stage-1

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

On Fri, Jun 26, 2026 at 10:35:40AM -0700, Oliver Upton wrote:
> On Fri, Jun 26, 2026 at 04:49:36PM +0100, Leonardo Bras wrote:
> > On Tue, Jun 23, 2026 at 11:41:52AM -0700, Oliver Upton wrote:
> > > Mark the descriptor as dirty if the permissions are sufficient to grant
> > > write access. Note that compute_s1_direct_permissions() already
> > > considers the DBM bit as writable.
> > 
> > IIUC the idea here is to avoid having a write fault when we first write to 
> > that page. Is that correct?
> > 
> > When we have HAFDBS the fault does not happen, and it gets marked from 
> > writeable-clean -> writeable-dirty by HAFDBS mechanism, which is cheap as 
> > it happens to be a write to the pgtable when memory is first written to.
> > 
> > On the other hand, when we clean the dirty-bits for the first time, we 
> > have to issue a write for every page that have been dirtied, in sequence.
> > 
> > An alternative would be not marking every page as dirty at first, in 
> > exchange of possibly having a faster first clean pass.
> > What do you think about it?
> 
> As Marc mentioned on the cover letter, this is unrelated to host dirty
> tracking. I'm implementing HAFDBS as part of our pseudo-TLB refill for
> nested. We need to mark translations as dirty in the nested S2 before
> filling the shadow S2 with something that allows writes.
> 

I see. Makes sense to me now.

Thanks for your patience!
Leo