Using __native_set_fixmap() here means guaranteed trap-and-emulate
instances the hypervisor has to deal with. Since the virtual address
covered by the to be adjusted page table entry is easy to determine (and
actually already gets obtained in a special case), simply use an
available, easy to invoke hypercall instead.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c
@@ -2010,6 +2010,7 @@ static unsigned char dummy_mapping[PAGE_
 static void xen_set_fixmap(unsigned idx, phys_addr_t phys, pgprot_t prot)
 {
 	pte_t pte;
+	unsigned long vaddr;
 
 	phys >>= PAGE_SHIFT;
 
@@ -2050,15 +2051,15 @@ static void xen_set_fixmap(unsigned idx,
 		break;
 	}
 
-	__native_set_fixmap(idx, pte);
+	vaddr = __fix_to_virt(idx);
+	if (HYPERVISOR_update_va_mapping(vaddr, pte, UVMF_INVLPG))
+		BUG();
 
 #ifdef CONFIG_X86_VSYSCALL_EMULATION
 	/* Replicate changes to map the vsyscall page into the user
 	   pagetable vsyscall mapping. */
-	if (idx == VSYSCALL_PAGE) {
-		unsigned long vaddr = __fix_to_virt(idx);
+	if (idx == VSYSCALL_PAGE)
 		set_pte_vaddr_pud(level3_user_vsyscall, vaddr, pte);
-	}
 #endif
 }