KVM: Remove dirty handling from gfn_to_pfn_cache completely

It isn't OK to cache the dirty status of a page in internal structures
for an indefinite period of time.

Any time a vCPU exits the run loop to userspace might be its last; the
VMM might do its final check of the dirty log, flush the last remaining
dirty pages to the destination and complete a live migration. If we
have internal 'dirty' state which doesn't get flushed until the vCPU
is finally destroyed on the source after migration is complete, then
we have lost data because that will escape the final copy.

This problem already exists with the use of kvm_vcpu_unmap() to mark
pages dirty in e.g. VMX nesting.

Note that the actual Linux MM already considers the page to be dirty
since we have a writeable mapping of it. This is just about the KVM
dirty logging.

For the nesting-style use cases (KVM_GUEST_USES_PFN) we will need to
track which gfn_to_pfn_caches have been used and explicitly mark the
corresponding pages dirty before returning to userspace. But we would
have needed external tracking of that anyway, rather than walking the
full list of GPCs to find those belonging to this vCPU which are dirty.

So let's rely *solely* on that external tracking, and keep it simple
rather than laying a tempting trap for callers to fall into.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220303154127.202856-3-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 5 insertions, 10 deletions

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index be9bbc0c6200..3f9b22c4983a 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1239,7 +1239,6 @@ void kvm_vcpu_mark_page_dirty(struct kvm_vcpu *vcpu, gfn_t gfn);
  *		   by KVM (and thus needs a kernel virtual mapping).
  * @gpa:	   guest physical address to map.
  * @len:	   sanity check; the range being access must fit a single page.
- * @dirty:         mark the cache dirty immediately.
  *
  * @return:	   0 for success.
  *		   -EINVAL for a mapping which would cross a page boundary.
@@ -1252,7 +1251,7 @@ void kvm_vcpu_mark_page_dirty(struct kvm_vcpu *vcpu, gfn_t gfn);
  */
 int kvm_gfn_to_pfn_cache_init(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
 			      struct kvm_vcpu *vcpu, enum pfn_cache_usage usage,
-			      gpa_t gpa, unsigned long len, bool dirty);
+			      gpa_t gpa, unsigned long len);
 
 /**
  * kvm_gfn_to_pfn_cache_check - check validity of a gfn_to_pfn_cache.
@@ -1261,7 +1260,6 @@ int kvm_gfn_to_pfn_cache_init(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
  * @gpc:	   struct gfn_to_pfn_cache object.
  * @gpa:	   current guest physical address to map.
  * @len:	   sanity check; the range being access must fit a single page.
- * @dirty:         mark the cache dirty immediately.
  *
  * @return:	   %true if the cache is still valid and the address matches.
  *		   %false if the cache is not valid.
@@ -1283,7 +1281,6 @@ bool kvm_gfn_to_pfn_cache_check(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
  * @gpc:	   struct gfn_to_pfn_cache object.
  * @gpa:	   updated guest physical address to map.
  * @len:	   sanity check; the range being access must fit a single page.
- * @dirty:         mark the cache dirty immediately.
  *
  * @return:	   0 for success.
  *		   -EINVAL for a mapping which would cross a page boundary.
@@ -1296,7 +1293,7 @@ bool kvm_gfn_to_pfn_cache_check(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
  * with the lock still held to permit access.
  */
 int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
-				 gpa_t gpa, unsigned long len, bool dirty);
+				 gpa_t gpa, unsigned long len);
 
 /**
  * kvm_gfn_to_pfn_cache_unmap - temporarily unmap a gfn_to_pfn_cache.
@@ -1304,10 +1301,9 @@ int kvm_gfn_to_pfn_cache_refresh(struct kvm *kvm, struct gfn_to_pfn_cache *gpc,
  * @kvm:	   pointer to kvm instance.
  * @gpc:	   struct gfn_to_pfn_cache object.
  *
- * This unmaps the referenced page and marks it dirty, if appropriate. The
- * cache is left in the invalid state but at least the mapping from GPA to
- * userspace HVA will remain cached and can be reused on a subsequent
- * refresh.
+ * This unmaps the referenced page. The cache is left in the invalid state
+ * but at least the mapping from GPA to userspace HVA will remain cached
+ * and can be reused on a subsequent refresh.
  */
 void kvm_gfn_to_pfn_cache_unmap(struct kvm *kvm, struct gfn_to_pfn_cache *gpc);
 
diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
index 784f37cbf33e..ac1ebb37a0ff 100644
--- a/include/linux/kvm_types.h
+++ b/include/linux/kvm_types.h
@@ -74,7 +74,6 @@ struct gfn_to_pfn_cache {
 	enum pfn_cache_usage usage;
 	bool active;
 	bool valid;
-	bool dirty;
 };
 
 #ifdef KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE