xen: add xen_set_domain_pte()

Add xen_set_domain_pte() to allow setting a pte mapping a page from
another domain.  The common case is to map from DOMID_IO, the pseudo
domain which owns all IO pages, but will also be used in the privcmd
interface to map other domain pages.

[ Impact: new Xen-internal API for cross-domain mappings ]
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

arch/x86/include/asm/xen/page.h

@@ -159,6 +159,7 @@ static inline pte_t __pte_ma(pteval_t x)
 
 #define pgd_val_ma(x)	((x).pgd)
 
+void xen_set_domain_pte(pte_t *ptep, pte_t pteval, unsigned domid);
 
 xmaddr_t arbitrary_virt_to_machine(void *address);
 unsigned long arbitrary_virt_to_mfn(void *vaddr);

arch/x86/xen/mmu.c

@@ -395,7 +395,7 @@ static bool xen_iomap_pte(pte_t pte)
 	return pte_flags(pte) & _PAGE_IOMAP;
 }
 
-static void xen_set_iomap_pte(pte_t *ptep, pte_t pteval)
+void xen_set_domain_pte(pte_t *ptep, pte_t pteval, unsigned domid)
 {
 	struct multicall_space mcs;
 	struct mmu_update *u;
@@ -407,10 +407,16 @@ static void xen_set_iomap_pte(pte_t *ptep, pte_t pteval)
 	u->ptr = arbitrary_virt_to_machine(ptep).maddr;
 	u->val = pte_val_ma(pteval);
 
-	MULTI_mmu_update(mcs.mc, mcs.args, 1, NULL, DOMID_IO);
+	MULTI_mmu_update(mcs.mc, mcs.args, 1, NULL, domid);
 
 	xen_mc_issue(PARAVIRT_LAZY_MMU);
 }
+EXPORT_SYMBOL_GPL(xen_set_domain_pte);
+
+static void xen_set_iomap_pte(pte_t *ptep, pte_t pteval)
+{
+	xen_set_domain_pte(ptep, pteval, DOMID_IO);
+}
 
 static void xen_extend_mmu_update(const struct mmu_update *update)
 {