[Xen-devel] [PATCH 2/2] dom_cow is needed for mem-sharing only

["Jan Beulich" <JBeulich@xxxxxxxx>]

A couple of adjustments are needed to code checking for dom_cow, but
since there are pretty few it is probably better to adjust those than
to set up and keep around a never used domain.

Take the opportunity and tighten a BUG_ON() in emul-priv-op.c:read_cr().
(Arguably this perhaps shouldn't be a BUG_ON() in the first place.)

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
While for now this avoids creating the domain on Arm only, Tamas'es
patch switching to CONFIG_MEM_SHARING will make x86 leverage this too.

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -967,8 +967,8 @@ get_page_from_l1e(
         return flip;
     }
 
-    if ( unlikely( (real_pg_owner != pg_owner) &&
-                   (real_pg_owner != dom_cow) ) )
+    if ( unlikely((real_pg_owner != pg_owner) &&
+                  (!dom_cow || (real_pg_owner != dom_cow))) )
     {
         /*
          * Let privileged domains transfer the right to map their target
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -568,7 +568,8 @@ struct page_info *p2m_get_page_from_gfn(
             }
             else if ( !get_page(page, p2m->domain) &&
                       /* Page could be shared */
-                      (!p2m_is_shared(*t) || !get_page(page, dom_cow)) )
+                      (!dom_cow || !p2m_is_shared(*t) ||
+                       !get_page(page, dom_cow)) )
                 page = NULL;
         }
         p2m_read_unlock(p2m);
@@ -941,7 +942,8 @@ guest_physmap_add_entry(struct domain *d
     /* Then, look for m->p mappings for this range and deal with them */
     for ( i = 0; i < (1UL << page_order); i++ )
     {
-        if ( page_get_owner(mfn_to_page(mfn_add(mfn, i))) == dom_cow )
+        if ( dom_cow &&
+             page_get_owner(mfn_to_page(mfn_add(mfn, i))) == dom_cow )
         {
             /* This is no way to add a shared page to your physmap! */
             gdprintk(XENLOG_ERR, "Adding shared mfn %lx directly to dom%d 
physmap not allowed.\n",
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -723,8 +723,8 @@ static int read_cr(unsigned int reg, uns
             unmap_domain_page(pl4e);
             *val = compat_pfn_to_cr3(mfn_to_gmfn(currd, mfn_x(mfn)));
         }
-        /* PTs should not be shared */
-        BUG_ON(page_get_owner(mfn_to_page(mfn)) == dom_cow);
+        /* PTs should be owned by their domains */
+        BUG_ON(page_get_owner(mfn_to_page(mfn)) != currd);
         return X86EMUL_OKAY;
     }
     }
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -74,7 +74,9 @@ integer_param("hardware_dom", hardware_d
 /* Private domain structs for DOMID_XEN, DOMID_IO, etc. */
 struct domain *__read_mostly dom_xen;
 struct domain *__read_mostly dom_io;
+#ifdef CONFIG_HAS_MEM_SHARING
 struct domain *__read_mostly dom_cow;
+#endif
 
 struct vcpu *idle_vcpu[NR_CPUS] __read_mostly;
 
@@ -544,12 +546,14 @@ void __init setup_special_domains(void)
     dom_io = domain_create(DOMID_IO, NULL, false);
     BUG_ON(IS_ERR(dom_io));
 
+#ifdef CONFIG_HAS_MEM_SHARING
     /*
      * Initialise our COW domain.
      * This domain owns sharable pages.
      */
     dom_cow = domain_create(DOMID_COW, NULL, false);
     BUG_ON(IS_ERR(dom_cow));
+#endif
 }
 
 void domain_update_node_affinity(struct domain *d)
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -1095,7 +1095,7 @@ map_grant_ref(
             host_map_created = true;
         }
     }
-    else if ( owner == rd || owner == dom_cow )
+    else if ( owner == rd || (dom_cow && owner == dom_cow) )
     {
         if ( (op->flags & GNTMAP_device_map) && !(op->flags & GNTMAP_readonly) 
)
         {
--- a/xen/include/xen/mm.h
+++ b/xen/include/xen/mm.h
@@ -644,6 +644,9 @@ static inline void filtered_flush_tlb_ma
 
 /* Private domain structs for DOMID_XEN, DOMID_IO, etc. */
 extern struct domain *dom_xen, *dom_io, *dom_cow;
+#ifndef CONFIG_HAS_MEM_SHARING
+# define dom_cow NULL
+#endif
 
 enum XENSHARE_flags {
     SHARE_rw,




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