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

[Tian Zheng]

On 10/29/2026 7:54 PM, Leonardo Bras wrote:
> On Thu, Oct 29, 2026 at 10:54:24AM +0800, Tian Zheng wrote:
> > Hi Leo,
> >
> > I've read your discussion with Oliver on DBM RFC v2. As a suggestion,
> > maybe we should go with Oliver's approach, i.e. revert to the v1 scheme
> > and continue fixing from there.
>
> Hi Tian,
> I have been reading a bit more about permission indirection to try figuring
> out a way that the change may work for both ways.
>
> The problem with the previous approach is that it assume DBM=write at all
> times, which is not architecturally correct. Having DBM=write only on
> HAFDBS=on as suggested would require us to transverse the whole pagetable
> to convert encodings when we enable/disable it, which can be quite
> expensive.
>
> One option would be to always enable HAFDBS when system supports HDBSS, but
> I still have to figure out if that causes any performance regression before
> suggesting.
>
> The option I am pursuing is thinking on an unified code path for direct
> permissison (both HACDBS=on and HACDBS=off) so we don't have to convert
> between them, neither worry about the impact of HAFDBS=on on performance.
>
> I will reply on the thread once I figure one of those options out.
>
> Thanks!
> Leo

Hi Leo,

Sounds good. I'll wait for your next version and review it once it's
posted.

Thanks,
Tian