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

[Tian Zheng]

On 10/23/2026 8:01 PM, Leonardo Bras wrote:
> 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
> 
Hi Leo,

OK, I've seen the new DBM RFC patchset. HDBSS v6 will be based on it.

Thanks,
Tian