KVM: arm64: Document why pause cannot be turned into a flag

It would be tempting to turn the 'pause' state into a flag.

However, this cannot easily be done as it is updated out of context,
while all the flags expect to only be updated from the vcpu thread.
Turning it into a flag would require to make all flag updates
atomic, which isn't necessary desireable.

Document this, and take this opportunity to move the field next
to the flag sets, filling a hole in the vcpu structure.

Reviewed-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/include/asm/kvm_host.h

@@ -341,6 +341,15 @@ struct kvm_vcpu_arch {
 	/* State flags for kernel bookkeeping, unused by the hypervisor code */
 	u8 sflags;
 
+	/*
+	 * Don't run the guest (internal implementation need).
+	 *
+	 * Contrary to the flags above, this is set/cleared outside of
+	 * a vcpu context, and thus cannot be mixed with the flags
+	 * themselves (or the flag accesses need to be made atomic).
+	 */
+	bool pause;
+
 	/*
 	 * We maintain more than a single set of debug registers to support
 	 * debugging the guest from the host and to maintain separate host and
@@ -394,9 +403,6 @@ struct kvm_vcpu_arch {
 	/* vcpu power state */
 	struct kvm_mp_state mp_state;
 
-	/* Don't run the guest (internal implementation need) */
-	bool pause;
-
 	/* Cache some mmu pages needed inside spinlock regions */
 	struct kvm_mmu_memory_cache mmu_page_cache;