It's currently only used for XPTI.  Move the code to a separate helper in
preparation for it gaining more logic.

While there switch to using l4e_write(): in the current context the L4 is
not active when modified, but that could change.

No functional change intended.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
 xen/arch/x86/domain.c         | 4 +---
 xen/arch/x86/include/asm/mm.h | 3 +++
 xen/arch/x86/mm.c             | 7 +++++++
 3 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 3d3c14dbb5ae..9cfcf0dc63f3 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1918,9 +1918,7 @@ void cf_check paravirt_ctxt_switch_to(struct vcpu *v)
     root_pgentry_t *root_pgt = this_cpu(root_pgt);
 
     if ( root_pgt )
-        root_pgt[root_table_offset(PERDOMAIN_VIRT_START)] =
-            l4e_from_page(v->domain->arch.perdomain_l3_pg,
-                          __PAGE_HYPERVISOR_RW);
+        setup_perdomain_slot(v, root_pgt);
 
     if ( unlikely(v->arch.dr7 & DR7_ACTIVE_MASK) )
         activate_debugregs(v);
diff --git a/xen/arch/x86/include/asm/mm.h b/xen/arch/x86/include/asm/mm.h
index 076e7009dc99..2c309f7b1444 100644
--- a/xen/arch/x86/include/asm/mm.h
+++ b/xen/arch/x86/include/asm/mm.h
@@ -630,4 +630,7 @@ static inline bool arch_mfns_in_directmap(unsigned long mfn, unsigned long nr)
     return (mfn + nr) <= (virt_to_mfn(eva - 1) + 1);
 }
 
+/* Setup the per-domain slot in the root page table pointer. */
+void setup_perdomain_slot(const struct vcpu *v, root_pgentry_t *root_pgt);
+
 #endif /* __ASM_X86_MM_H__ */
diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index a1ac7bdc5b44..35e929057d21 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -6362,6 +6362,13 @@ unsigned long get_upper_mfn_bound(void)
     return min(max_mfn, 1UL << (paddr_bits - PAGE_SHIFT)) - 1;
 }
 
+void setup_perdomain_slot(const struct vcpu *v, root_pgentry_t *root_pgt)
+{
+    l4e_write(&root_pgt[root_table_offset(PERDOMAIN_VIRT_START)],
+              l4e_from_page(v->domain->arch.perdomain_l3_pg,
+                            __PAGE_HYPERVISOR_RW));
+}
+
 static void __init __maybe_unused build_assertions(void)
 {
     /*
-- 
2.45.2