xen: Hide get_phys_to_machine() to be able to tune common path

Today get_phys_to_machine() is always called when the mfn for a pfn
is to be obtained. Add a wrapper __pfn_to_mfn() as inline function
to be able to avoid calling get_phys_to_machine() when possible as
soon as the switch to a linear mapped p2m list has been done.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

arch/x86/xen/p2m.c

@@ -787,7 +787,7 @@ static int m2p_add_override(unsigned long mfn, struct page *page,
 	 * because mfn_to_pfn (that ends up being called by GUPF) will
 	 * return the backend pfn rather than the frontend pfn. */
 	pfn = mfn_to_pfn_no_overrides(mfn);
-	if (get_phys_to_machine(pfn) == mfn)
+	if (__pfn_to_mfn(pfn) == mfn)
 		set_phys_to_machine(pfn, FOREIGN_FRAME(mfn));
 
 	return 0;
@@ -967,7 +967,7 @@ static int m2p_remove_override(struct page *page,
 	 * pfn again. */
 	mfn &= ~FOREIGN_FRAME_BIT;
 	pfn = mfn_to_pfn_no_overrides(mfn);
-	if (get_phys_to_machine(pfn) == FOREIGN_FRAME(mfn) &&
+	if (__pfn_to_mfn(pfn) == FOREIGN_FRAME(mfn) &&
 			m2p_find_override(mfn) == NULL)
 		set_phys_to_machine(pfn, mfn);
 
@@ -992,7 +992,7 @@ int clear_foreign_p2m_mapping(struct gnttab_unmap_grant_ref *unmap_ops,
 	}
 
 	for (i = 0; i < count; i++) {
-		unsigned long mfn = get_phys_to_machine(page_to_pfn(pages[i]));
+		unsigned long mfn = __pfn_to_mfn(page_to_pfn(pages[i]));
 		unsigned long pfn = page_to_pfn(pages[i]);
 
 		if (mfn == INVALID_P2M_ENTRY || !(mfn & FOREIGN_FRAME_BIT)) {