An optimization was added to avic_vcpu_load() in commit 782f64558de7
("KVM: SVM: Skip AVIC and IRTE updates when loading blocking vCPU")
to avoid re-enabling AVIC if the vCPU is about to block.
Such situation arises when a vCPU thread is preempted in between the call
to kvm_arch_vcpu_blocking() and before the matching call to
kvm_arch_vcpu_unblocking() which in case of AVIC disables/enables the AVIC
on this vCPU.
The same optimization was done in avic_vcpu_put() however the code was
based on physical id table's 'is_running' bit, building upon assumption
that if avic_vcpu_load() didn't set it, then kvm doesn't need to disable
avic (since it wasn't really enabled).
However, once AVIC's IPI virtualization is made optional, this bit
might be always false regardless if a vCPU is running or not.
To fix this, instead of checking this bit, check the same
'kvm_vcpu_is_blocking()' condition.
Also, as a bonus, re-enable the warning for already set 'is_running' bit,
if it was found set, during avic_vcpu_put() execution and the vCPU was not
blocking a condition which indicates a bug.
Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
---
arch/x86/kvm/svm/avic.c | 15 ++++++---------
1 file changed, 6 insertions(+), 9 deletions(-)
diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 2092db892d7d052..4c75ca15999fcd4 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -1075,16 +1075,10 @@ void avic_vcpu_put(struct kvm_vcpu *vcpu)
lockdep_assert_preemption_disabled();
/*
- * Note, reading the Physical ID entry outside of ir_list_lock is safe
- * as only the pCPU that has loaded (or is loading) the vCPU is allowed
- * to modify the entry, and preemption is disabled. I.e. the vCPU
- * can't be scheduled out and thus avic_vcpu_{put,load}() can't run
- * recursively.
+ * If the vcpu is blocking, there is no need to do anything.
+ * See the comment in avic_vcpu_load().
*/
- entry = READ_ONCE(*(svm->avic_physical_id_cache));
-
- /* Nothing to do if IsRunning == '0' due to vCPU blocking. */
- if (!(entry & AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK))
+ if (kvm_vcpu_is_blocking(vcpu))
return;
/*
@@ -1099,6 +1093,9 @@ void avic_vcpu_put(struct kvm_vcpu *vcpu)
avic_update_iommu_vcpu_affinity(vcpu, -1, 0);
+ entry = READ_ONCE(*(svm->avic_physical_id_cache));
+ WARN_ON_ONCE(!(entry & AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK));
+
entry &= ~AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK;
WRITE_ONCE(*(svm->avic_physical_id_cache), entry);
--
2.26.3
