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

[Leonardo Bras]

On Fri, Oct 30, 2026 at 02:01:49PM +0800, Tian Zheng wrote:
> 
> 
> 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

Awesome, thanks!
Leo