[PATCH v2 18/19] KVM: arm64: Document why pause cannot be turned into a flag

Marc Zyngier
2022-06-10T09:35:40Z

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>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
arch/arm64/include/asm/kvm_host.h | 12 +++++++++---
1 file 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
@@ -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;

--
2.34.1