[PATCH v8 06/29] KVM: arm64: Introduce non-UNDEF FGT control

[Mark Brown]

On Fri, Sep 19, 2025 at 04:14:49PM +0100, Marc Zyngier wrote:

> FGUs are uniform, because when something doesn't exist on a vcpu, it
> doesn't exist on *any* vcpu. Non-FGU use of FGTs, however, has to be
> more flexible because that's part of the emulation, and is actually
> pretty rare that we want to trap something at all times, on all vcpus.

> For the same reason, conflating the R and W registers doesn't work
> either. For the above example, I want to be able to trap write
> accesses to MDSCR_EL1, and not reads, just like the Ampere
> brain-damage.

> So please make this per-vcpu, decouple R and W FGTs, and convert the
> Ampere horror to this scheme.

OK, that makes more sense - it was a bit confusing that all the FGT
handling was done per VM not per vCPU without even any provision for
per-vCPU or distinct R/W stuff, it seemed strange.  Since the SME
requirement also ends up being per VM anyway I just tried to fit in with
what was there but if we don't want that behaviour it's certainly more
obvious to make it per-vCPU and to split read and write.
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