[v3,1/4] x86/shadow: replace sh_reset_l3_up_pointers()

Commit Message

Jan Beulich Feb. 8, 2023, 2:37 p.m. UTC

Rather than doing a separate hash walk (and then even using the vCPU
variant, which is to go away), do the up-pointer-clearing right in
sh_unpin(), as an alternative to the (now further limited) enlisting on
a "free floating" list fragment. This utilizes the fact that such list
fragments are traversed only for multi-page shadows (in shadow_free()
and sh_next_page()). Furthermore sh_terminate_list() is a safe guard
only anyway, which isn't in use in the common case (it actually does
something only for BIGMEM configurations).

Signed-off-by: Jan Beulich <[e-mail]>
---
v3: Clear sp->up uniformly for all L3/64 shadows.

Comments

Andrew Cooper Feb. 9, 2023, 5:28 p.m. UTC | #1

On 08/02/2023 2:37 pm, Jan Beulich wrote:
> Rather than doing a separate hash walk (and then even using the vCPU
> variant, which is to go away), do the up-pointer-clearing right in
> sh_unpin(), as an alternative to the (now further limited) enlisting on
> a "free floating" list fragment. This utilizes the fact that such list
> fragments are traversed only for multi-page shadows (in shadow_free()
> and sh_next_page()). Furthermore sh_terminate_list() is a safe guard
> only anyway, which isn't in use in the common case (it actually does
> something only for BIGMEM configurations).
>
> Signed-off-by: Jan Beulich <[e-mail]>

Acked-by: Andrew Cooper <[e-mail]>

Patch

--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -2371,29 +2371,6 @@  static void sh_detach_old_tables(struct
 
 /**************************************************************************/
 
-/* Reset the up-pointers of every L3 shadow to 0.
- * This is called when l3 shadows stop being pinnable, to clear out all
- * the list-head bits so the up-pointer field is properly inititalised. */
-static int cf_check sh_clear_up_pointer(
-    struct vcpu *v, mfn_t smfn, mfn_t unused)
-{
-    mfn_to_page(smfn)->up = 0;
-    return 0;
-}
-
-void sh_reset_l3_up_pointers(struct vcpu *v)
-{
-    static const hash_vcpu_callback_t callbacks[SH_type_unused] = {
-        [SH_type_l3_64_shadow] = sh_clear_up_pointer,
-    };
-
-    HASH_CALLBACKS_CHECK(SHF_L3_64);
-    hash_vcpu_foreach(v, SHF_L3_64, callbacks, INVALID_MFN);
-}
-
-
-/**************************************************************************/
-
 static void sh_update_paging_modes(struct vcpu *v)
 {
     struct domain *d = v->domain;
--- a/xen/arch/x86/mm/shadow/multi.c
+++ b/xen/arch/x86/mm/shadow/multi.c
@@ -971,7 +971,6 @@  sh_make_shadow(struct vcpu *v, mfn_t gmf
                     sh_unpin(d, page_to_mfn(sp));
             }
             d->arch.paging.shadow.opt_flags &= ~SHOPT_LINUX_L3_TOPLEVEL;
-            sh_reset_l3_up_pointers(v);
         }
     }
 #endif
--- a/xen/arch/x86/mm/shadow/private.h
+++ b/xen/arch/x86/mm/shadow/private.h
@@ -505,11 +505,6 @@  void shadow_blow_tables(struct domain *d
  */
 int sh_remove_all_mappings(struct domain *d, mfn_t gmfn, gfn_t gfn);
 
-/* Reset the up-pointers of every L3 shadow to 0.
- * This is called when l3 shadows stop being pinnable, to clear out all
- * the list-head bits so the up-pointer field is properly inititalised. */
-void sh_reset_l3_up_pointers(struct vcpu *v);
-
 /******************************************************************************
  * Flags used in the return value of the shadow_set_lXe() functions...
  */
@@ -729,7 +724,7 @@  static inline void sh_unpin(struct domai
 {
     struct page_list_head tmp_list, *pin_list;
     struct page_info *sp, *next;
-    unsigned int i, head_type;
+    unsigned int i, head_type, sz;
 
     ASSERT(mfn_valid(smfn));
     sp = mfn_to_page(smfn);
@@ -741,20 +736,29 @@  static inline void sh_unpin(struct domai
         return;
     sp->u.sh.pinned = 0;
 
-    /* Cut the sub-list out of the list of pinned shadows,
-     * stitching it back into a list fragment of its own. */
+    sz = shadow_size(head_type);
+
+    /*
+     * Cut the sub-list out of the list of pinned shadows, stitching
+     * multi-page shadows back into a list fragment of their own.
+     */
     pin_list = &d->arch.paging.shadow.pinned_shadows;
     INIT_PAGE_LIST_HEAD(&tmp_list);
-    for ( i = 0; i < shadow_size(head_type); i++ )
+    for ( i = 0; i < sz; i++ )
     {
         ASSERT(sp->u.sh.type == head_type);
         ASSERT(!i || !sp->u.sh.head);
         next = page_list_next(sp, pin_list);
         page_list_del(sp, pin_list);
-        page_list_add_tail(sp, &tmp_list);
+        if ( sz > 1 )
+            page_list_add_tail(sp, &tmp_list);
+        else if ( head_type == SH_type_l3_64_shadow )
+            sp->up = 0;
         sp = next;
     }
-    sh_terminate_list(&tmp_list);
+
+    if ( sz > 1 )
+        sh_terminate_list(&tmp_list);
 
     sh_put_ref(d, smfn, 0);
 }