KVM: arm64: Remove ad-hoc CPTR manipulation from fpsimd_sve_sync()

There's no need for fpsimd_sve_sync() to write to CPTR/CPACR. All
relevant traps are always disabled earlier within __kvm_vcpu_run(), when
__deactivate_cptr_traps() configures CPTR/CPACR.

With irrelevant details elided, the flow is:

handle___kvm_vcpu_run(...)
{
	flush_hyp_vcpu(...) {
		fpsimd_sve_flush(...);
	}

	__kvm_vcpu_run(...) {
		__activate_traps(...) {
			__activate_cptr_traps(...);
		}

		do {
			__guest_enter(...);
		} while (...);

		__deactivate_traps(....) {
			__deactivate_cptr_traps(...);
		}
	}

	sync_hyp_vcpu(...) {
		fpsimd_sve_sync(...);
	}
}

Remove the unnecessary write to CPTR/CPACR. An ISB is still necessary,
so a comment is added to describe this requirement.

Signed-off-by: Mark Rutland <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Fuad Tabba <[email protected]>
Cc: Marc Zyngier <[email protected]>
Cc: Mark Brown <[email protected]>
Cc: Oliver Upton <[email protected]>
Cc: Will Deacon <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Marc Zyngier <[email protected]>

Author: mrutland-arm

arch/arm64/kvm/hyp/nvhe/hyp-main.c

@@ -69,7 +69,10 @@ static void fpsimd_sve_sync(struct kvm_vcpu *vcpu)
 	if (!guest_owns_fp_regs())
 		return;
 
-	cpacr_clear_set(0, CPACR_EL1_FPEN | CPACR_EL1_ZEN);
+	/*
+	 * Traps have been disabled by __deactivate_cptr_traps(), but there
+	 * hasn't necessarily been a context synchronization event yet.
+	 */
 	isb();
 
 	if (vcpu_has_sve(vcpu))