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

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 [off-list ref] wrote:

quoted

Hi Marc,

On Mon, Jan 19, 2026 at 10:56:45AM +0000, Marc Zyngier wrote:

quoted

[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