Re: [PATCH v4 3/6] KVM: arm64: Add auto DBM support for hardware dirty tracking

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

On Tue, Jul 14, 2026 at 03:44:37PM +0800, Tian Zheng wrote:
> 
> On 7/14/2026 3:23 PM, Marc Zyngier wrote:
> > On Tue, 14 Jul 2026 02:14:45 +0100,
> > Tian Zheng <zhengtian10@huawei.com> wrote:
> > > 
> > > On 7/13/2026 7:17 PM, Leonardo Bras wrote:
> > > > On Thu, Jul 09, 2026 at 06:40:23PM +0800, Tian Zheng wrote:
> > > > > The DBM (Dirty Bit Modifier) attribute, introduced in ARMv8.1, enables
> > > > > hardware to automatically promote write-clean pages to write-dirty. This
> > > > > prevents the guest from being trapped in EL2 due to missing write
> > > > > permissions.
> > > > > 
> > > > > In this design, DBM is controlled by the page-table level flag
> > > > > KVM_PGTABLE_S2_DBM rather than per-PTE software flags. DBM is
> > > > > automatically set for writable non-device pages when the page-table has
> > > > > KVM_PGTABLE_S2_DBM flag, which is determined at MMU init time based on
> > > > > hardware capability.
> > > > > 
> > > > > The DBM bit is set in stage2_set_prot_attr() for initial mappings and
> > > > > hugepage splitting, and directly manipulated in
> > > > > kvm_pgtable_stage2_relax_perms() when removing write-protection. On
> > > > > W->RO downgrade, DBM is cleared to prevent hardware from silently
> > > > > upgrading RO+DBM back to W+dirty, which would bypass KVM's write
> > > > > tracking.
> > > > > 
> > > > > kvm_pgtable_stage2_pte_prot() does not extract the DBM bit back into
> > > > > enum kvm_pgtable_prot because DBM is a page-table policy determined by
> > > > > pgt->flags, not a per-PTE property. Callers should check
> > > > > pgt->flags & KVM_PGTABLE_S2_DBM instead.
> > > > > 
> > > > > This ensures DBM is consistently applied across all PTEs, including
> > > > > during hugepage splitting where child PTEs inherit DBM from the parent
> > > > > block entry via the pgt->flags mechanism.
> > > > > 
> > > > > Safety: DBM bit is only interpreted by hardware when VTCR_EL2.HD=1.
> > > > > When HDBSS is not enabled (HD=0), ARM architecture guarantees hardware
> > > > > completely ignores DBM bit in PTEs.
> > > > > 
> > > > > Co-developed-by: Eillon <yezhenyu2@huawei.com>
> > > > > Signed-off-by: Eillon <yezhenyu2@huawei.com>
> > > > > Co-developed-by: Leonardo Bras <leo.bras@arm.com>
> > > > > Signed-off-by: Leonardo Bras <leo.bras@arm.com>
> > > > Hello Tian,
> > > > 
> > > > Have you added the above tags due to this patch being based on the below?
> > > > https://lore.kernel.org/all/20260629111820.1873540-2-leo.bras@arm.com/
> > > > 
> > > > Thanks!
> > > > Leo
> > > Hi Leo,
> > > 
> > > 
> > > Yes, I added your Signed-off-by because the DBM-related code in this patch
> > You really can't do that. Only Leo can give his SoB, you can't forge
> > it yourself.
> > 
> > > is based on your implementation in:
> > > 
> > > https://lore.kernel.org/all/20260629111820.1873540-2-leo.bras@arm.com/
> > Then take the patch as is, and add to it as a separate patch. Or work
> > out in private with Leo whether he's happy with a Co-dev. But never do
> > that unilaterally.
> > 
> > Thanks,
> > 
> > 	M.
> Got it, thanks for the clarification, Marc.
> 
> I'm sorry about that — I'll remove them in the next version.
> Instead, I'll mention the reference in the commit message with a link to
> Leo's original patch:
> 
> Based on Leonardo Bras's patch:
> https://lore.kernel.org/all/20260629111820.1873540-2-leo.bras@arm.com/

That works out for me!

Thanks!
Leo