[PATCH v5 3/6] x86: extend mod_l*_entry() to optionally return the old PTE value

[Kevin Lampis <kevin.lampis@xxxxxxxxxx>]

Calling mod_l*_entry() with a valid ol*e_out pointer will do atomic
CMPXCHG to set the new pte value and return the old pte value through
the ol*e_out pointer. If the ol*e_out pointer is NULL then old behavior
is preserved.

Signed-off-by: Kevin Lampis <kevin.lampis@xxxxxxxxxx>
---
Changes in v5:
- Refactoring the mod_l*_entry() functions was split out
  into a patch before this one

Changes in v4:
- Refactor how UPDATE_ENTRY is used
- Update commit message to specify CMPXCHG instead of xchg

Changes in v3:
- Add swap to mod_l{2,3,4}_entry()

Changes in v2:
- UPDATE_ENTRY now returns the old pte value instead of using an out
  pointer
---
 xen/arch/x86/mm.c | 77 ++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 59 insertions(+), 18 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index dc0e0d451c02..316a435b9b71 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -2193,7 +2193,8 @@ static void l3t_unlock(struct page_info *page)
 /* Update the L1 entry at pl1e to new value nl1e. */
 static int mod_l1_entry(l1_pgentry_t *pl1e, l1_pgentry_t nl1e,
                         mfn_t gl1mfn, unsigned int update_flags,
-                        struct vcpu *pt_vcpu, struct domain *pg_dom)
+                        struct vcpu *pt_vcpu, struct domain *pg_dom,
+                        l1_pgentry_t *ol1e_out)
 {
     l1_pgentry_t ol1e = l1e_read(pl1e);
     struct domain *pt_dom = pt_vcpu->domain;
@@ -2254,7 +2255,11 @@ static int mod_l1_entry(l1_pgentry_t *pl1e, l1_pgentry_t 
nl1e,
         /* Fast path for sufficiently-similar mappings. */
         if ( !l1e_has_changed(ol1e, nl1e, ~FASTPATH_FLAG_WHITELIST) )
         {
-            UPDATE_ENTRY(l1, pl1e, ol1e, nl1e, gl1mfn, pt_vcpu, update_flags);
+            intpte_t old = UPDATE_ENTRY(l1, pl1e, ol1e, nl1e, gl1mfn, pt_vcpu,
+                                        update_flags);
+            if ( ol1e_out )
+                *ol1e_out = l1e_from_intpte(old);
+
             if ( page )
                 put_page(page);
             return 0;
@@ -2280,7 +2285,12 @@ static int mod_l1_entry(l1_pgentry_t *pl1e, l1_pgentry_t 
nl1e,
     else if ( pv_l1tf_check_l1e(pt_dom, nl1e) )
         return -ERESTART;
 
-    UPDATE_ENTRY(l1, pl1e, ol1e, nl1e, gl1mfn, pt_vcpu, update_flags);
+    {
+        intpte_t old = UPDATE_ENTRY(l1, pl1e, ol1e, nl1e, gl1mfn, pt_vcpu,
+                                    update_flags);
+        if ( ol1e_out )
+            *ol1e_out = l1e_from_intpte(old);
+    }
 
     put_page_from_l1e(ol1e, pt_dom);
     return 0;
@@ -2292,7 +2302,8 @@ static int mod_l2_entry(l2_pgentry_t *pl2e,
                         l2_pgentry_t nl2e,
                         mfn_t mfn,
                         unsigned int update_flags,
-                        struct vcpu *vcpu)
+                        struct vcpu *vcpu,
+                        l2_pgentry_t *ol2e_out)
 {
     l2_pgentry_t ol2e;
     struct domain *d = vcpu->domain;
@@ -2324,7 +2335,11 @@ static int mod_l2_entry(l2_pgentry_t *pl2e,
         /* Fast path for sufficiently-similar mappings. */
         if ( !l2e_has_changed(ol2e, nl2e, ~FASTPATH_PDE_FLAG_WHITELIST) )
         {
-            UPDATE_ENTRY(l2, pl2e, ol2e, nl2e, mfn, vcpu, update_flags);
+            intpte_t old = UPDATE_ENTRY(l2, pl2e, ol2e, nl2e, mfn, vcpu,
+                                        update_flags);
+            if ( ol2e_out )
+                *ol2e_out = l2e_from_intpte(old);
+
             return 0;
         }
 
@@ -2334,7 +2349,12 @@ static int mod_l2_entry(l2_pgentry_t *pl2e,
     else if ( pv_l1tf_check_l2e(d, nl2e) )
         return -ERESTART;
 
-    UPDATE_ENTRY(l2, pl2e, ol2e, nl2e, mfn, vcpu, update_flags);
+    {
+        intpte_t old = UPDATE_ENTRY(l2, pl2e, ol2e, nl2e, mfn, vcpu,
+                                    update_flags);
+        if ( ol2e_out )
+            *ol2e_out = l2e_from_intpte(old);
+    }
 
     put_page_from_l2e(ol2e, mfn, PTF_defer);
 
@@ -2346,7 +2366,8 @@ static int mod_l3_entry(l3_pgentry_t *pl3e,
                         l3_pgentry_t nl3e,
                         mfn_t mfn,
                         unsigned int update_flags,
-                        struct vcpu *vcpu)
+                        struct vcpu *vcpu,
+                        l3_pgentry_t *ol3e_out)
 {
     l3_pgentry_t ol3e;
     struct domain *d = vcpu->domain;
@@ -2376,7 +2397,11 @@ static int mod_l3_entry(l3_pgentry_t *pl3e,
         /* Fast path for sufficiently-similar mappings. */
         if ( !l3e_has_changed(ol3e, nl3e, ~FASTPATH_PDE_FLAG_WHITELIST) )
         {
-            UPDATE_ENTRY(l3, pl3e, ol3e, nl3e, mfn, vcpu, update_flags);
+            intpte_t old = UPDATE_ENTRY(l3, pl3e, ol3e, nl3e, mfn, vcpu,
+                                        update_flags);
+            if ( ol3e_out )
+                *ol3e_out = l3e_from_intpte(old);
+
             return 0;
         }
 
@@ -2387,7 +2412,12 @@ static int mod_l3_entry(l3_pgentry_t *pl3e,
     else if ( pv_l1tf_check_l3e(d, nl3e) )
         return -ERESTART;
 
-    UPDATE_ENTRY(l3, pl3e, ol3e, nl3e, mfn, vcpu, update_flags);
+    {
+        intpte_t old = UPDATE_ENTRY(l3, pl3e, ol3e, nl3e, mfn, vcpu,
+                                    update_flags);
+        if ( ol3e_out )
+            *ol3e_out = l3e_from_intpte(old);
+    }
 
     put_page_from_l3e(ol3e, mfn, PTF_defer);
     return 0;
@@ -2398,7 +2428,8 @@ static int mod_l4_entry(l4_pgentry_t *pl4e,
                         l4_pgentry_t nl4e,
                         mfn_t mfn,
                         unsigned int update_flags,
-                        struct vcpu *vcpu)
+                        struct vcpu *vcpu,
+                        l4_pgentry_t *ol4e_out)
 {
     struct domain *d = vcpu->domain;
     l4_pgentry_t ol4e;
@@ -2428,7 +2459,11 @@ static int mod_l4_entry(l4_pgentry_t *pl4e,
         /* Fast path for sufficiently-similar mappings. */
         if ( !l4e_has_changed(ol4e, nl4e, ~FASTPATH_PDE_FLAG_WHITELIST) )
         {
-            UPDATE_ENTRY(l4, pl4e, ol4e, nl4e, mfn, vcpu, update_flags);
+            intpte_t old = UPDATE_ENTRY(l4, pl4e, ol4e, nl4e, mfn, vcpu,
+                                        update_flags);
+            if ( ol4e_out )
+                *ol4e_out = l4e_from_intpte(old);
+
             return 0;
         }
 
@@ -2439,7 +2474,12 @@ static int mod_l4_entry(l4_pgentry_t *pl4e,
     else if ( pv_l1tf_check_l4e(d, nl4e) )
         return -ERESTART;
 
-    UPDATE_ENTRY(l4, pl4e, ol4e, nl4e, mfn, vcpu, update_flags);
+    {
+        intpte_t old = UPDATE_ENTRY(l4, pl4e, ol4e, nl4e, mfn, vcpu,
+                                    update_flags);
+        if ( ol4e_out )
+            *ol4e_out = l4e_from_intpte(old);
+    }
 
     put_page_from_l4e(ol4e, mfn, PTF_defer);
     return 0;
@@ -3311,7 +3351,7 @@ int new_guest_cr3(mfn_t mfn)
                           l4e_from_mfn(mfn,
                                        (_PAGE_PRESENT | _PAGE_RW |
                                         _PAGE_USER | _PAGE_ACCESSED)),
-                          gt_mfn, 0, curr);
+                          gt_mfn, 0, curr, NULL);
         unmap_domain_page(pl4e);
         switch ( rc )
         {
@@ -4149,14 +4189,14 @@ long do_mmu_update(
                 {
                 case PGT_l1_page_table:
                     rc = mod_l1_entry(va, l1e_from_intpte(req.val), mfn,
-                                      update_flags, v, pg_owner);
+                                      update_flags, v, pg_owner, NULL);
                     break;
 
                 case PGT_l2_page_table:
                     if ( unlikely(pg_owner != pt_owner) )
                         break;
                     rc = mod_l2_entry(va, l2e_from_intpte(req.val), mfn,
-                                      update_flags, v);
+                                      update_flags, v, NULL);
                     if ( !rc )
                         flush_linear_pt = true;
                     break;
@@ -4165,7 +4205,7 @@ long do_mmu_update(
                     if ( unlikely(pg_owner != pt_owner) )
                         break;
                     rc = mod_l3_entry(va, l3e_from_intpte(req.val), mfn,
-                                      update_flags, v);
+                                      update_flags, v, NULL);
                     if ( !rc )
                         flush_linear_pt = true;
                     break;
@@ -4174,7 +4214,7 @@ long do_mmu_update(
                     if ( unlikely(pg_owner != pt_owner) )
                         break;
                     rc = mod_l4_entry(va, l4e_from_intpte(req.val), mfn,
-                                      update_flags, v);
+                                      update_flags, v, NULL);
                     if ( !rc )
                         flush_linear_pt = true;
                     if ( !rc && pt_owner->arch.pv.xpti )
@@ -4514,7 +4554,8 @@ static int __do_update_va_mapping(
         goto out;
     }
 
-    rc = mod_l1_entry(pl1e, val, gl1mfn, MMU_NORMAL_PT_UPDATE, v, pg_owner);
+    rc = mod_l1_entry(pl1e, val, gl1mfn, MMU_NORMAL_PT_UPDATE, v, pg_owner,
+                      NULL);
 
     page_unlock(gl1pg);
     put_page(gl1pg);
-- 
2.52.0