[xen master] xen/gnttab: reduce size of struct active_grant_entry

[patchbot@xxxxxxx]

commit a2f520314169fdaa47ab19b2ca8fb9d9f1554a36
Author:     Juergen Gross <jgross@xxxxxxxx>
AuthorDate: Fri Dec 2 10:28:33 2022 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Dec 2 10:28:33 2022 +0100

    xen/gnttab: reduce size of struct active_grant_entry
    
    The size of struct active_grant_entry for 64-bit builds is 40 or 48
    bytes today (with or without NDEBUG).
    
    It can easily be reduced by 8 bytes by replacing the trans_domain
    pointer with the domid of the related domain. trans_domain is only ever
    used for transitive grants, which doesn't have any known users.
    
    This reduction will result in less memory usage and (for production
    builds) in faster code, as indexing into the active_grant_entry array
    will be much easier with an entry having a power-of-2 size.
    
    The performance loss when using transitive grants shouldn't really
    matter, given the probability that those aren't in use at all.
    
    Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/common/grant_table.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index fa28c3cb7b..b896f9af0e 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -381,13 +381,13 @@ struct active_grant_entry {
 })
 
     domid_t       domid;  /* Domain being granted access.             */
+    domid_t       src_domid; /* Original domain granting access.      */
     unsigned int  start:15; /* For sub-page grants, the start offset
                                in the page.                           */
     bool          is_sub_page:1; /* True if this is a sub-page grant. */
     unsigned int  length:16; /* For sub-page grants, the length of the
                                 grant.                                */
     grant_ref_t   trans_gref;
-    struct domain *trans_domain;
     mfn_t         mfn;    /* Machine frame being granted.             */
 #ifndef NDEBUG
     gfn_t         gfn;    /* Guest's idea of the frame being granted. */
@@ -1095,7 +1095,7 @@ map_grant_ref(
             act->start = 0;
             act->length = PAGE_SIZE;
             act->is_sub_page = false;
-            act->trans_domain = rd;
+            act->src_domid = rd->domain_id;
             act->trans_gref = ref;
         }
     }
@@ -2497,7 +2497,8 @@ release_grant_for_copy(
     else
     {
         status = &status_entry(rgt, gref);
-        td = act->trans_domain;
+        td = (act->src_domid == rd->domain_id)
+             ? rd : knownalive_domain_from_domid(act->src_domid);
         trans_gref = act->trans_gref;
     }
 
@@ -2659,7 +2660,7 @@ acquire_grant_for_copy(
                           !mfn_eq(act->mfn, grant_mfn) ||
                           act->start != trans_page_off ||
                           act->length != trans_length ||
-                          act->trans_domain != td ||
+                          act->src_domid != td->domain_id ||
                           act->trans_gref != trans_gref ||
                           !act->is_sub_page)) )
         {
@@ -2692,7 +2693,7 @@ acquire_grant_for_copy(
             act->domid = ldom;
             act->start = trans_page_off;
             act->length = trans_length;
-            act->trans_domain = td;
+            act->src_domid = td->domain_id;
             act->trans_gref = trans_gref;
             act->mfn = grant_mfn;
             act_set_gfn(act, INVALID_GFN);
@@ -2754,7 +2755,7 @@ acquire_grant_for_copy(
             act->is_sub_page = is_sub_page;
             act->start = trans_page_off;
             act->length = trans_length;
-            act->trans_domain = td;
+            act->src_domid = td->domain_id;
             act->trans_gref = trans_gref;
             act->mfn = grant_mfn;
         }
--
generated by git-patchbot for /home/xen/git/xen.git#master