KVM: Move x86's API to release a faultin page to common KVM

Move KVM x86's helper that "finishes" the faultin process to common KVM
so that the logic can be shared across all architectures.  Note, not all
architectures implement a fast page fault path, but the gist of the
comment applies to all architectures.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20241010182427.1434605-50-seanjc@google.com>

1 files changed, 26 insertions, 0 deletions

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 6efdc00b4254..3e06393e5f1e 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1231,6 +1231,32 @@ static inline void kvm_release_page_unused(struct page *page)
 void kvm_release_page_clean(struct page *page);
 void kvm_release_page_dirty(struct page *page);
 
+static inline void kvm_release_faultin_page(struct kvm *kvm, struct page *page,
+					    bool unused, bool dirty)
+{
+	lockdep_assert_once(lockdep_is_held(&kvm->mmu_lock) || unused);
+
+	if (!page)
+		return;
+
+	/*
+	 * If the page that KVM got from the *primary MMU* is writable, and KVM
+	 * installed or reused a SPTE, mark the page/folio dirty.  Note, this
+	 * may mark a folio dirty even if KVM created a read-only SPTE, e.g. if
+	 * the GFN is write-protected.  Folios can't be safely marked dirty
+	 * outside of mmu_lock as doing so could race with writeback on the
+	 * folio.  As a result, KVM can't mark folios dirty in the fast page
+	 * fault handler, and so KVM must (somewhat) speculatively mark the
+	 * folio dirty if KVM could locklessly make the SPTE writable.
+	 */
+	if (unused)
+		kvm_release_page_unused(page);
+	else if (dirty)
+		kvm_release_page_dirty(page);
+	else
+		kvm_release_page_clean(page);
+}
+
 kvm_pfn_t __kvm_faultin_pfn(const struct kvm_memory_slot *slot, gfn_t gfn,
 			    unsigned int foll, bool *writable,
 			    struct page **refcounted_page);