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>

1 files changed, 9 insertions, 3 deletions

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index c6975ecf5a5f..2cc42e1fec18 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -342,6 +342,15 @@ struct kvm_vcpu_arch {
 	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
 	 * guest state during world switches. vcpu_debug_state are the debug
@@ -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;