[PATCH] Use cmpxchg for pte updates on walk_addr()

[Marcelo Tosatti <marcelo-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org>]

In preparation for multi-threaded guest pte walking, use cmpxchg()
when updating guest pte's. This guarantees that the assignment of the
dirty bit can't be lost if two CPU's are faulting the same address
simultaneously.

In case of pte update via write emulation, no synchronization is
necessary since its the guest responsability.

Signed-off-by: Marcelo Tosatti <mtosatti-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>


diff --git a/drivers/kvm/paging_tmpl.h b/drivers/kvm/paging_tmpl.h
index b24bc7c..593073a 100644
--- a/drivers/kvm/paging_tmpl.h
+++ b/drivers/kvm/paging_tmpl.h
@@ -78,6 +78,23 @@ static gfn_t gpte_to_gfn_pde(pt_element_t gpte)
 	return (gpte & PT_DIR_BASE_ADDR_MASK) >> PAGE_SHIFT;
 }
 
+static inline void FNAME(kvm_cmpxchg_guest_pte)(struct kvm *kvm,
+			 gfn_t table_gfn, unsigned index, pt_element_t new_bits)
+{
+	pt_element_t pte;
+	pt_element_t *table;
+	struct page *page;
+
+	page = gfn_to_page(kvm, table_gfn);
+	table = kmap_atomic(page, KM_USER0);
+
+	do {
+ 		pte = table[index];
+	} while (cmpxchg(&table[index], pte, pte | new_bits) != pte);
+
+	kunmap_atomic(page, KM_USER0);
+}
+
 /*
  * Fetch a guest pte for a guest virtual address
  */
@@ -136,7 +153,8 @@ static int FNAME(walk_addr)(struct guest_walker *walker,
 		if (!(pte & PT_ACCESSED_MASK)) {
 			mark_page_dirty(vcpu->kvm, table_gfn);
 			pte |= PT_ACCESSED_MASK;
-			kvm_write_guest(vcpu->kvm, pte_gpa, &pte, sizeof(pte));
+			FNAME(kvm_cmpxchg_guest_pte)
+				(vcpu->kvm, table_gfn, index, pte);
 		}
 
 		if (walker->level == PT_PAGE_TABLE_LEVEL) {
@@ -161,7 +179,7 @@ static int FNAME(walk_addr)(struct guest_walker *walker,
 	if (write_fault && !is_dirty_pte(pte)) {
 		mark_page_dirty(vcpu->kvm, table_gfn);
 		pte |= PT_DIRTY_MASK;
-		kvm_write_guest(vcpu->kvm, pte_gpa, &pte, sizeof(pte));
+		FNAME(kvm_cmpxchg_guest_pte)(vcpu->kvm, table_gfn, index, pte);
 		kvm_mmu_pte_write(vcpu, pte_gpa, (u8 *)&pte, sizeof(pte));
 	}
 
diff --git a/drivers/kvm/x86.c b/drivers/kvm/x86.c
index c70ac33..edba8ce 100644
--- a/drivers/kvm/x86.c
+++ b/drivers/kvm/x86.c
@@ -1510,6 +1510,9 @@ static int emulator_write_phys(struct kvm_vcpu *vcpu, gpa_t gpa,
 {
 	int ret;
 
+	/* No need for kvm_cmpxchg_guest_pte here, its the guest 
+ 	 * responsability to synchronize pte updates and page faults.
+	 */
 	ret = kvm_write_guest(vcpu->kvm, gpa, val, bytes);
 	if (ret < 0)
 		return 0;

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