KVM: Questions and comments on make_all_cpus_request

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Hi KVM guys,

I've had a bit of challenges figuring out the exact functinality and
synchronization of vcpu->requests and friends.  In lack of a better
method, I wrote some comments as a patch.

I think this code really deserves some explaining, as it is really hard
to understand otherwise.  Unfortunately, I wasn't able to write down
concise and exact comments, but I hope someone else feels up to the
challenge.

Let me know if I just got this completely wrong and upside down.

Thanks,
Christoffer
---
 include/linux/kvm_host.h |    4 ++++
 virt/kvm/kvm_main.c      |   29 +++++++++++++++++++++++++++--
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index cbe0d68..25deef8 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -252,6 +252,10 @@ struct kvm_vcpu {
 	struct kvm_vcpu_arch arch;
 };
 
+/*
+ * XXX: Could we explain what we're trying to achieve? Is this an
+ * optimization as to not send multiple IPIs?
+ */
 static inline int kvm_vcpu_exiting_guest_mode(struct kvm_vcpu *vcpu)
 {
 	return cmpxchg(&vcpu->mode, IN_GUEST_MODE, EXITING_GUEST_MODE);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index e45c20c..ccc292d 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -165,7 +165,18 @@ static void ack_flush(void *_completed)
 {
 }
 
-static bool make_all_cpus_request(struct kvm *kvm, unsigned int req)
+/**
+ * make_all_cpus_request - place request on vcpus
+ * @kvm: KVM Struct
+ * @req: Request to make on the VCPU
+ *
+ * This function places a request on a VCPU and ensures that the VCPU request
+ * is handled before returning from the function, if the VCPU is in guest
+ * mode (or exiting, or reading shadow page tables?).
+ *
+ * Returns true if at least of the vcpus were sent an IPI and responded to it,
+ */
+static bool make_all_vcpus_request(struct kvm *kvm, unsigned int req)
 {
 	int i, cpu, me;
 	cpumask_var_t cpus;
@@ -179,9 +190,19 @@ static bool make_all_cpus_request(struct kvm *kvm, unsigned int req)
 		kvm_make_request(req, vcpu);
 		cpu = vcpu->cpu;
 
+		/*
+		 * Is the following really true? Can we have an example of the
+		 * race that would otherwise happen? Doesn't the make_request
+		 * pair against the IPI and interrupt disabling, not the mode?
+		 */
 		/* Set ->requests bit before we read ->mode */
 		smp_mb();
 
+		/*
+		 * Set the bit on the CPU mask for all CPUs which are somehow
+		 * running a guest (IN_GUEST_MODE, EXITING_GUEST_MODE, and
+		 * READING_SHADOW_PAGE_TABLES).
+		 */
 		if (cpus != NULL && cpu != -1 && cpu != me &&
 		      kvm_vcpu_exiting_guest_mode(vcpu) != OUTSIDE_GUEST_MODE)
 			cpumask_set_cpu(cpu, cpus);
@@ -201,9 +222,13 @@ void kvm_flush_remote_tlbs(struct kvm *kvm)
 {
 	long dirty_count = kvm->tlbs_dirty;
 
-	smp_mb();
+	smp_mb(); /* TODO: Someone should explain this! */
 	if (make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH))
 		++kvm->stat.remote_tlb_flush;
+	/*
+	 * TODO: Someone should explain this, why is it a cmpxchg, what
+	 * happens if the dirty is different from dirty_count?
+	 */
 	cmpxchg(&kvm->tlbs_dirty, dirty_count, 0);
 }
 
-- 
1.7.9.5

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