[PATCH v4 08/12] KVM: x86/xen: Explicitly tag "shared info" page as never being dirty tracked

[David Woodhouse <dwmw2@infradead.org>]

From: Sean Christopherson <seanjc@google.com>

Explicitly mark the Xen shared info page as never being dirty tracked so
that higher-level gpc APIs can be added to automatically take care of
things like dirty tracking, without reintroducing the bug fixed by
commit 55749769fe60 ("KVM: x86: Fix wall clock writes in Xen shared_info
not to mark page dirty").  And because the code _looks_ buggy.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Assisted-by: LLM
---
 arch/x86/kvm/xen.c        |  2 +-
 include/linux/kvm_host.h  | 15 +++++++++++----
 include/linux/kvm_types.h |  1 +
 virt/kvm/pfncache.c       |  4 +++-
 4 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 6a1162caee9b..0ba684bf01d3 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -2577,7 +2577,7 @@ void kvm_xen_init_vm(struct kvm *kvm)
 {
 	mutex_init(&kvm->arch.xen.xen_lock);
 	xa_init(&kvm->arch.xen.evtchn_ports);
-	kvm_gpc_init(&kvm->arch.xen.shinfo_cache, kvm);
+	__kvm_gpc_init(&kvm->arch.xen.shinfo_cache, kvm, true);
 }
 
 void kvm_xen_destroy_vm(struct kvm *kvm)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 94989a705ee6..ba23f4e0a4c0 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1456,16 +1456,23 @@ int kvm_vcpu_write_guest(struct kvm_vcpu *vcpu, gpa_t gpa, const void *data,
 			 unsigned long len);
 
 /**
- * kvm_gpc_init - initialize gfn_to_pfn_cache.
+ * __kvm_gpc_init - initialize gfn_to_pfn_cache.
  *
  * @gpc:	   struct gfn_to_pfn_cache object.
  * @kvm:	   pointer to kvm instance.
+ * @never_dirty:   the cache never needs to mark its page dirty in its memslot.
  *
  * This sets up a gfn_to_pfn_cache by initializing locks and assigning the
  * immutable attributes.  Note, the cache must be zero-allocated (or zeroed by
- * the caller before init).
+ * the caller before init).  Most callers want the kvm_gpc_init() wrapper.
  */
-void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm);
+void __kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm,
+		    bool never_dirty);
+
+static inline void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm)
+{
+	__kvm_gpc_init(gpc, kvm, false);
+}
 
 /**
  * kvm_gpc_activate - prepare a cached kernel mapping and HPA for a given guest
@@ -1990,7 +1997,7 @@ static inline void kvm_gpc_mark_dirty_in_slot(struct gfn_to_pfn_cache *gpc)
 {
 	lockdep_assert_held(&gpc->lock);
 
-	if (!gpc->memslot)
+	if (!gpc->memslot || gpc->never_dirty)
 		return;
 
 	mark_page_dirty_in_slot(gpc->kvm, gpc->memslot, gpa_to_gfn(gpc->gpa));
diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
index a568d8e6f4e8..e850adc3f47e 100644
--- a/include/linux/kvm_types.h
+++ b/include/linux/kvm_types.h
@@ -94,6 +94,7 @@ struct gfn_to_pfn_cache {
 	kvm_pfn_t pfn;
 	bool active;
 	bool valid;
+	bool never_dirty;
 };
 
 #ifdef KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE
diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 0e700c337f5a..8498abb746ad 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -437,7 +437,8 @@ int kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, unsigned long len, bool wait)
 	}
 }
 
-void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm)
+void __kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm,
+		    bool never_dirty)
 {
 	rwlock_init(&gpc->lock);
 	mutex_init(&gpc->refresh_lock);
@@ -447,6 +448,7 @@ void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm)
 	gpc->gpa = INVALID_GPA;
 	gpc->uhva = KVM_HVA_ERR_BAD;
 	gpc->active = gpc->valid = false;
+	gpc->never_dirty = never_dirty;
 }
 
 static int __kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long uhva,
-- 
2.43.0