[PATCH 3/4] x86: extend do_mmu_update() to support returning the old PTE value

[Kevin Lampis <kevin.lampis@xxxxxxxxxx>]

A new parameter write_back_old when set will return the old PTE value.

- The new PTE value in req.val must be 0, this is for clearing only.

- The PRESERVE_AD flag is rejected with -EINVAL because preserving
  Accessed/Dirty bits into a zero'ed PTE doesn't make sense.

- Only l1 PTEs are supported because they are the most frequent and have the
  biggest performance impact.

- The old PTE value is passed back to the guest through the req.val field

If the write_back_old flag is not set then the old behavior is preserved
  do_mmu_update -> mod_l1_entry -> UPDATE_ENTRY -> paging_write_guest_entry

The new get_and_clear call chain looks like this
  do_mmu_update -> mod_l1_entry -> update_intpte -> paging_cmpxchg_guest_entry

Signed-off-by: Kevin Lampis <kevin.lampis@xxxxxxxxxx>
---
 xen/arch/x86/mm.c | 62 ++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 58 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 500549c8e036..278b992aca5c 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -3988,11 +3988,12 @@ long do_mmuext_op(
     return rc;
 }
 
-long do_mmu_update(
+static long __do_mmu_update(
     XEN_GUEST_HANDLE_PARAM(mmu_update_t) ureqs,
     unsigned int count,
     XEN_GUEST_HANDLE_PARAM(uint) pdone,
-    unsigned int foreigndom)
+    unsigned int foreigndom,
+    bool write_back_old)
 {
     struct mmu_update req;
     void *va = NULL;
@@ -4144,13 +4145,41 @@ long do_mmu_update(
                 switch ( page->u.inuse.type_info & PGT_type_mask )
                 {
                 case PGT_l1_page_table:
-                    rc = mod_l1_entry(va, l1e_from_intpte(req.val), mfn,
-                                      cmd, v, pg_owner, NULL);
+                {
+                    if ( !write_back_old )
+                        rc = mod_l1_entry(va, l1e_from_intpte(req.val), mfn,
+                                          cmd, v, pg_owner, NULL);
+                    else
+                    {
+                        l1_pgentry_t ol1e;
+                        if ( unlikely(req.val != 0 ||
+                                      cmd == MMU_PT_UPDATE_PRESERVE_AD) )
+                        {
+                            rc = -EINVAL;
+                            break;
+                        }
+
+                        rc = mod_l1_entry(va, l1e_from_intpte(req.val), mfn,
+                                          cmd, v, pg_owner, &ol1e);
+
+                        if ( !rc )
+                        {
+                            req.val = ol1e.l1;
+                            if ( unlikely(copy_to_guest(ureqs, &req, 1)) )
+                                rc = -EFAULT;
+                        }
+                    }
                     break;
+                }
 
                 case PGT_l2_page_table:
                     if ( unlikely(pg_owner != pt_owner) )
                         break;
+                    if ( unlikely(write_back_old) )
+                    {
+                        rc = -EINVAL;
+                        break;
+                    }
                     rc = mod_l2_entry(va, l2e_from_intpte(req.val), mfn,
                                       cmd == MMU_PT_UPDATE_PRESERVE_AD, v);
                     if ( !rc )
@@ -4160,6 +4189,11 @@ long do_mmu_update(
                 case PGT_l3_page_table:
                     if ( unlikely(pg_owner != pt_owner) )
                         break;
+                    if ( unlikely(write_back_old) )
+                    {
+                        rc = -EINVAL;
+                        break;
+                    }
                     rc = mod_l3_entry(va, l3e_from_intpte(req.val), mfn,
                                       cmd == MMU_PT_UPDATE_PRESERVE_AD, v);
                     if ( !rc )
@@ -4169,6 +4203,11 @@ long do_mmu_update(
                 case PGT_l4_page_table:
                     if ( unlikely(pg_owner != pt_owner) )
                         break;
+                    if ( unlikely(write_back_old) )
+                    {
+                        rc = -EINVAL;
+                        break;
+                    }
                     rc = mod_l4_entry(va, l4e_from_intpte(req.val), mfn,
                                       cmd == MMU_PT_UPDATE_PRESERVE_AD, v);
                     if ( !rc )
@@ -4198,6 +4237,11 @@ long do_mmu_update(
                     break;
 
                 case PGT_writable_page:
+                    if ( unlikely(write_back_old) )
+                    {
+                        rc = -EINVAL;
+                        break;
+                    }
                     perfc_incr(writable_mmu_updates);
                     paging_write_guest_entry(v, va, req.val, mfn);
                     rc = 0;
@@ -4366,6 +4410,16 @@ long do_mmu_update(
 
     return rc;
 }
+
+long do_mmu_update(
+    XEN_GUEST_HANDLE_PARAM(mmu_update_t) ureqs,
+    unsigned int count,
+    XEN_GUEST_HANDLE_PARAM(uint) pdone,
+    unsigned int foreigndom)
+{
+    return __do_mmu_update(ureqs, count, pdone, foreigndom, false);
+}
+
 #endif /* CONFIG_PV */
 
 /*
-- 
2.52.0