[RFC PATCH v2 06/10] kvm: gmem: add tracepoints for gmem share/unshare

[Patrick Roy]

Add tracepoints for calls to kvm_gmem_get_folio that cause the returned
folio to be considered "shared" (e.g. accessible by host KVM), and
tracepoint for when KVM is done accessing a gmem pfn
(kvm_gmem_put_shared_pfn).

The above operations can cause folios to be insert/removed into/from the
direct map. We want to be able to make sure that only those gmem folios
that we expect KVM to access are ever reinserted into the direct map,
and that all folios that are temporarily reinserted are also removed
again at a later point. Processing ftrace output is one way to verify
this.

Signed-off-by: Patrick Roy <roypat@xxxxxxxxxxxx>
---
 include/trace/events/kvm.h | 43 ++++++++++++++++++++++++++++++++++++++
 virt/kvm/guest_memfd.c     |  7 ++++++-
 2 files changed, 49 insertions(+), 1 deletion(-)

diff --git a/include/trace/events/kvm.h b/include/trace/events/kvm.h
index 74e40d5d4af42..4a40fd4c22f91 100644
--- a/include/trace/events/kvm.h
+++ b/include/trace/events/kvm.h
@@ -489,6 +489,49 @@ TRACE_EVENT(kvm_test_age_hva,
 	TP_printk("mmu notifier test age hva: %#016lx", __entry->hva)
 );
 
+#ifdef CONFIG_KVM_PRIVATE_MEM
+TRACE_EVENT(kvm_gmem_share,
+	TP_PROTO(struct folio *folio, pgoff_t index),
+	TP_ARGS(folio, index),
+
+	TP_STRUCT__entry(
+		__field(unsigned int, sharing_count)
+		__field(kvm_pfn_t, pfn)
+		__field(pgoff_t, index)
+		__field(unsigned long,  npages)
+	),
+
+	TP_fast_assign(
+		__entry->sharing_count = refcount_read(folio_get_private(folio));
+		__entry->pfn = folio_pfn(folio);
+		__entry->index = index;
+		__entry->npages = folio_nr_pages(folio);
+	),
+
+	TP_printk("pfn=0x%llx index=%lu pages=%lu (refcount now %d)",
+	          __entry->pfn, __entry->index, __entry->npages, __entry->sharing_count - 1)
+);
+
+TRACE_EVENT(kvm_gmem_unshare,
+	TP_PROTO(kvm_pfn_t pfn),
+	TP_ARGS(pfn),
+
+	TP_STRUCT__entry(
+		__field(unsigned int, sharing_count)
+		__field(kvm_pfn_t, pfn)
+	),
+
+	TP_fast_assign(
+		__entry->sharing_count = refcount_read(folio_get_private(pfn_folio(pfn)));
+		__entry->pfn = pfn;
+	),
+
+	TP_printk("pfn=0x%llx (refcount now %d)",
+	          __entry->pfn, __entry->sharing_count - 1)
+)
+
+#endif
+
 #endif /* _TRACE_KVM_MAIN_H */
 
 /* This part must be outside protection */
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 6772253497e4d..742eba36d2371 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -7,6 +7,7 @@
 #include <linux/set_memory.h>
 
 #include "kvm_mm.h"
+#include "trace/events/kvm.h"
 
 struct kvm_gmem {
 	struct kvm *kvm;
@@ -204,8 +205,10 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index, unsi
 	if (r)
 		goto out_err;
 
-	if (share)
+	if (share) {
 		refcount_inc(folio_get_private(folio));
+		trace_kvm_gmem_share(folio, index);
+	}
 
 out:
 	/*
@@ -759,6 +762,8 @@ int kvm_gmem_put_shared_pfn(kvm_pfn_t pfn) {
 	if (refcount_read(sharing_count) == 1)
 		r = kvm_gmem_folio_set_private(folio);
 
+	trace_kvm_gmem_unshare(pfn);
+
 	return r;
 }
 EXPORT_SYMBOL_GPL(kvm_gmem_put_shared_pfn);
-- 
2.46.0