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

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

On Fri, Oct 23, 2026 at 03:23:04PM +0800, Tian Zheng wrote:
> 
> 
> On 10/14/2026 11:56 PM, Leonardo Bras wrote:
> > On Thu, Sep 17, 2026 at 02:40:42PM +0100, Leonardo Bras wrote:
> > > On Wed, Sep 16, 2026 at 04:27:19PM -0700, Oliver Upton wrote:
> > > > 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.
> > > > 
> > > 
> > > Okay then, I will get a machine to figure the performance gains :)
> > 
> > Got the performance numbers here.
> > I tested both the case where all PTEs are dirty, and the case that 50% of
> > the PTEs are dirty.
> > 
> > For that test, actually patch #5 could make kvm_pgtable_stage2_wrprotect()
> > slower, which is counter-intuitive, in all platforms tested:
> > 
> > When we walk the S2 pagetable to wrprotect, using the new walker, the
> > write to the PTE itself is so cheap* that the cost of the branch
> > misprediction (write PTE or not) for the 50/50 case is significantly higher
> > than the cost of the 50% extra writes, which makes the 50/50 up to 60%
> > slower than the 100% dirty case. It is still slower in the '25% dirty' as
> > well, and starts to get even at '10% dirty'.
> > 
> > * (non-atomic, everything is in order, walker is small, cpu will probably
> > bundle it all in a single cacheline write)
> > 
> > Since the idea of patch #5 was to reduce PTE writes in the first wrprotect
> > walk, it becomes faster only if only a small portion (<10%) of the VM was
> > write-faulted while the remaining was read-faulted, which seems to be a
> > really reduced audience.
> > 
> > Since this comes with increased complexity, and also carry costs of
> > having HAFDBS itself enabled outside dirty-tracking, I don't think it's a
> > good idea to further pursue patch #5. It's only value is that it served as
> > a base for HDBSS implementation.
> > 
> > Thanks for reading!
> > Leo
> > 
> Hi Leo,
> 
> Thanks for running this, the numbers are really useful.
> 
> It means the hardware dirty mode (HD=1) should only be enabled while
> dirty logging is active with HDBSS support, and stays off otherwise.

Hi Tian,

Correct, it's actually counter-productive for it to stay on without HDBSS 
before dirty-logging.

> 
> With HD only enabled during dirty logging, we also keep the DBM-based
> encoding for the HD=0 case. On systems that support FEAT_HAFDBS, DBM
> remains the write bit and S2AP[1] the dirty bit even with HD=0. This
> works because kvm_s2_fault_compute_prot() maps any writable page of a
> non-tracked memslot writable-dirty (WD), including on read faults:
> 
> ```c
> if (s2vi->map_writable) {
>         *prot |= KVM_PGTABLE_PROT_W;
> 
>         if (s2vi->device ||
>             !memslot_is_logging(s2fd->memslot) ||
>             kvm_is_write_fault(s2fd->vcpu))
>                 *prot |= KVM_PGTABLE_PROT_DIRTY;
> }
> ```
> 
> So there is no WC entry for hardware to promote, and HD is simply not
> needed outside dirty logging.
> 
> In the next HDBSS version, we'll only turn on HD=1 when dirty logging
> is active.
> 

Right! 
I will also send a new version of the pre-patch (4 patches) for us to 
discuss. The first 2 patches will change meaning a bit, but should still 
work as they do today.

Thanks!
Leo