Re: [PATCH 0/6] KVM: arm64: ... and FWB for all

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi Marc,

On Fri, Jan 23, 2026 at 12:37:17PM +0000, Marc Zyngier wrote:
> On Fri, 23 Jan 2026 12:22:58 +0000,
> Alexandru Elisei <alexandru.elisei@arm.com> wrote:
> > 
> > Hi Marc,
> > 
> > On Mon, Jan 19, 2026 at 10:56:45AM +0000, Marc Zyngier wrote:
> > > [Yes, you can tell what I was listening to]
> > > 
> > > Today, running in protected mode puts the host under it's own S2 with
> > > FWB=0, even if the rest of the guests are running with FWB=1. The
> > > rationale for this special-casing of the host is that we want the
> > > combined attributes to actually reflect the ones exposed by S1.
> > > 
> > > We therefore use NormalCacheable (which is the weakest memory type) as
> > > the default attributes at S2 so that S1 can only strengthen the final
> > > memory type.
> > > 
> > > But there is no reason why we cannot achieve the same effect with
> > > FWB. We normally use FWB to enforce cacheable memory from S2,
> > > irrespective of S1. But it is possible to configure the S2 attributes
> > > as "pass-through", so that the S1 attributes are always used.
> > 
> > Would you mind clarifying why FWB is preferable? Is it so that pkvm uses
> > the same approach for configuring stage 2 for the host as for a guest? Or
> > is it something else?
> 
> Having only one attribute encoding scheme to think about on a given
> machine is vastly preferable, specially when you are debugging.
> Additionally, FWB==0 may not be with us forever.

Got it, thanks for the explanation!

Thanks,
Alex