Re: [RFC PATCH 5/5] KVM: arm64: Enable HAFDBS for guests not on migration

[Oliver Upton]

On Wed, Sep 16, 2026 at 03:00:46PM +0100, Leonardo Bras wrote:
> On Tue, Sep 15, 2026 at 05:10:02PM -0700, Oliver Upton wrote:
> > Hi,
> > 
> > On Tue, Sep 01, 2026 at 06:15:56PM +0100, Leonardo Bras wrote:
> > > When dirty-logging is disabled, even non-write faults make a page dirty,
> > > which avoids a second fault when the page is actually written to.
> > > 
> > > On dirty-logging enable, this approach causes all (writable) pages on the
> > > memslot to be marked clean, even if they were not written to, which can
> > > take a lot of time, while holding the MMU lock, doing atomic writes to
> > > PTEs.
> > 
> > Do you have any performance numbers for this? Enabling HAFDBS seems a
> > bit involved to avoid some stores on the first pass.
> 
> Not yet, but if the idea does not look too crazy I can find hardware and 
> collect some data :)

TBH this looks like a micro-optimization so I'm not expecting the
performance gains to justify the behavior change.

> > > 						      has_vhe() &&
> > 
> > I don't see a reason why this needs to be constrained to VHE-only.
> > 
> 
> Humm, in nVHE would not the host kernel run in EL1?
> I thought that this being a feature that depends on EL2 registers host 
> would need to be in EL2 to make use of it.
> 
> That being said, I understand very little of how this works, so I 
> constrained to VHE only at the start.
> 
> Would this work in nVHE?

We already pass a stage-2 MMU configuration to EL2 from EL1 in nVHE and
hVHE. How is this any different?

I'm not opposed to making features VHE-only, but there needs to be some
amount of reasoning to justify it.

> > > +		!kvm_vcpu_has_nv(kvm) && cpus_have_final_cap(ARM64_HW_DBM);
> > 
> > Same thing goes for nested... KVM can make use of HAFDBS in the
> > canonical stage-2 MMU (or even a shadow stage-2) independent of the
> > guest hypervisor.
> > 
> 
> Humm, I remember reaching the conclusion that it could not be used if the 
> guest supported NV. Let's say:
> 
> L0 - Host	- Has HAFDBS enabled
> L1 - Hypervisor - Has HAFDBS disabled
> L2 - Guest	-
> 
> Let's say guest writes to a page, and the shadow S2 has DBM=1, so it's 
> marked as WD by HAFDBS. Since no fault was taken, how would the L1 be able 
> to update it's S2 pagetables to mark the page dirty?
> 
> (We would have to transverse the Shadow S2 Pagetable updating the original 
> S2 pagetable)
> 
> I was wondering, thought, that we could emulate it in the last level 
> hypervisor, if it's guest does not support nested guests. That would mean 
> we can have the last-1 level hypervisor to update the S2 pagetable on the 
> last level hypervisor without it having to fault. Ex:
> 
> L0 Host - HAFDBS disabled
> [...]
> Ln-1 Hypervisor - HAFDBS disabled
> Ln   Hypervisor - HAFDBS enabled
> Ln+1 Guest - No E2H feature
> 
> When the guest writes to a page, the host should receive a fault, that IIUC 
> have to propagate down up to Ln Hyp. If Ln Hyp has HAFDBS, we could skip 
> injecting a fault in Ln Hyp, as  Ln-1 Hyp could emulate HAFDBS and write 
> the dirty bit to S2 pagetagle of Ln+1 guest, that resides in Ln memory.
> 
> Not sure if the troulbe would be worth, though.
> Does it make sense?

I'm not following your reasoning here. Treat the shadow stage-2 MMU as a
TLB; that TLB is filled with a writable translation when S2AP[1]=1 in the
L1 translation.

The dirty state of the pseudo-TLB is completely internal. You could then
layer HAFDBS for the L1 translation on top of this (which we don't
support) by potentially relaxing the descriptor _before_ evaluating the
resulting permissions. You'd then take write permission faults to set
S2AP[1] in the L1 translation.

> > The name would suggest this thing takes a vcpu pointer...
> > 
> 
> Ah, that name was based on
> #define kvm_vcpu_has_feature(k, f)  __vcpu_has_feature(&(k)->arch, #(f))
> 
> That takes a kvm struct to check the kvm_arch one, instead of looking into 
> the vcpu. I did it like this because there were some scenarios it was not 
> quite straightforward to get the vcpu to use vcpu_has_nv(), which takes a 
> vcpu.

This thing probably should've been "kvm_has_vcpu_feature()" or similar to
massage the expected typing.

Thanks,
Oliver