KVM: arm64: nv: Don't save/restore FP register during a nested ERET or exception

When switching between L1 and L2, we save the old state using
kvm_arch_vcpu_put(), mutate the state in memory, then load the new
state using kvm_arch_vcpu_load(). Any live FPSIMD/SVE state is saved
and unbound, such that it can be lazily restored on a subsequent trap.

The FPSIMD/SVE state is shared by exception levels, and only a handful
of related control registers need to be changed when transitioning
between L1 and L2. The save/restore of the common state is needless
overhead, especially as trapping becomes exponentially more expensive
with nesting.

Avoid this overhead by leaving the common FPSIMD/SVE state live on the
CPU, and only switching the state that is distinct for L1 and L2:

- the trap controls: the effective values are recomputed on each entry
  into the guest to take the EL into account and merge the L0 and L1
  configuration if in a nested context, or directly use the L0 configuration
  in non-nested context (see __activate_traps()).

- the VL settings: the effective values are are also recomputed on each
  entry into the guest (see fpsimd_lazy_switch_to_guest()).

Since we appear to cover all bases, use the vcpu flags indicating the
handling of a nested ERET or exception delivery to avoid the whole FP
save/restore shenanigans. SME will have to be similarly dealt with when
it eventually gets supported.

For an EL1 L3 guest where L1 and L2 have this optimisation, this
results in at least a 10% wall clock reduction when running an I/O
heavy workload, generating a high rate of nested exceptions.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://patch.msgid.link/20260520085036.541666-3-maz@kernel.org

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