x86/vmx: use a MEMF_no_refcount domheap page for APIC_DEFAULT_PHYS_BASE

vmx_alloc_vlapic_mapping() currently contains some very odd looking code
that allocates a MEMF_no_owner domheap page and then shares with the guest
as if it were a xenheap page. This then requires vmx_free_vlapic_mapping()
to call a special function in the mm code: free_shared_domheap_page().

By using a MEMF_no_refcount domheap page instead, the odd looking code in
vmx_alloc_vlapic_mapping() can simply use get_page_and_type() to set up a
writable mapping before insertion in the P2M and vmx_free_vlapic_mapping()
can simply release the page using put_page_alloc_ref() followed by
put_page_and_type(). This then allows free_shared_domheap_page() to be
purged.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>

xen/arch/x86/hvm/vmx/vmx.c

@@ -3029,12 +3029,22 @@ static int vmx_alloc_vlapic_mapping(struct domain *d)
     if ( !cpu_has_vmx_virtualize_apic_accesses )
         return 0;
 
-    pg = alloc_domheap_page(d, MEMF_no_owner);
+    pg = alloc_domheap_page(d, MEMF_no_refcount);
     if ( !pg )
         return -ENOMEM;
+
+    if ( !get_page_and_type(pg, d, PGT_writable_page) )
+    {
+        /*
+         * The domain can't possibly know about this page yet, so failure
+         * here is a clear indication of something fishy going on.
+         */
+        domain_crash(d);
+        return -ENODATA;
+    }
+
     mfn = page_to_mfn(pg);
     clear_domain_page(mfn);
-    share_xen_page_with_guest(pg, d, SHARE_rw);
     d->arch.hvm.vmx.apic_access_mfn = mfn;
 
     return set_mmio_p2m_entry(d, paddr_to_pfn(APIC_DEFAULT_PHYS_BASE), mfn,
@@ -3048,7 +3058,12 @@ static void vmx_free_vlapic_mapping(struct domain *d)
 
     d->arch.hvm.vmx.apic_access_mfn = _mfn(0);
     if ( !mfn_eq(mfn, _mfn(0)) )
-        free_shared_domheap_page(mfn_to_page(mfn));
+    {
+        struct page_info *pg = mfn_to_page(mfn);
+
+        put_page_alloc_ref(pg);
+        put_page_and_type(pg);
+    }
 }
 
 static void vmx_install_vlapic_mapping(struct vcpu *v)

xen/arch/x86/mm.c

@@ -502,16 +502,6 @@ void share_xen_page_with_guest(struct page_info *page, struct domain *d,
     spin_unlock(&d->page_alloc_lock);
 }
 
-void free_shared_domheap_page(struct page_info *page)
-{
-    put_page_alloc_ref(page);
-    if ( !test_and_clear_bit(_PGC_xen_heap, &page->count_info) )
-        ASSERT_UNREACHABLE();
-    page->u.inuse.type_info = 0;
-    page_set_owner(page, NULL);
-    free_domheap_page(page);
-}
-
 void make_cr3(struct vcpu *v, mfn_t mfn)
 {
     struct domain *d = v->domain;

xen/include/asm-x86/mm.h

@@ -320,8 +320,6 @@ struct page_info
 
 #define maddr_get_owner(ma)   (page_get_owner(maddr_to_page((ma))))
 
-extern void free_shared_domheap_page(struct page_info *page);
-
 #define frame_table ((struct page_info *)FRAMETABLE_VIRT_START)
 extern unsigned long max_page;
 extern unsigned long total_pages;