Re: [PATCH 1/2] KVM: arm64: Hide FEAT_LS64_ACCDATA from guests

[Fuad Tabba]

Hi Marc,

On Fri, 23 Oct 2026 09:31:32 +0100, Marc Zyngier <maz@xxxxxxxxxx> wrote:
[...]
> Why settle for the least capable option? Context switching ACCDATA_EL1
> is easy enough, can be gated on guest features, and avoid the
> userspace horror story.
>
> I'd rather see that.

Context switching it means accessing ACCDATA_EL1 at EL2, which traps
to EL3 unless SCR_EL3.ADEn is set. booting.rst doesn't require ADEn
today, and TF-A only sets it when built with ENABLE_FEAT_LS64_ACCDATA,
which defaults to off. On such a host, KVM traps to EL3 the first time
it switches ACCDATA_EL1 for a VM, and every VM has LS64_ACCDATA by
default. The cap leaves the register alone, and the fine-grained trap
takes priority over ADEn, so it works whatever ADEn is set to.

Should we keep the cap as the fix for stable, and post ACCDATA support
on top? If you agree, I'd rebase your patches 3, 4 and 8 from [1], add
a booting.rst requirement for ADEn and EnAS0, and lift the cap and the
compat.

Cheers,
/fuad

[1] https://lore.kernel.org/all/20240815125959.2097734-1-maz@xxxxxxxxxx/