[PATCH] x86/pv: gate setting per-domain slot to XPTI being active for the domain

[Roger Pau Monne]

It's possible to have XPTI not active for all PV domains (active for domUs,
inactive for dom0), hence don't gate setting the per-domain slot on the
presence of the per-pCPU shadow root page-table.  Instead set the slot based on
whether XPTI is active for the domain.  This avoid pointlessly setting the
per-domain slot if the shadow root page-table won't be used by the domain.

Fixes: 0d3e7f0b6bf0 ('xen/x86: support per-domain flag for xpti')
Signed-off-by: Roger Pau Monné <roger....@citrix.com>
---
 xen/arch/x86/domain.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 78a13e6812c9..fd6bb3663027 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1902,12 +1902,11 @@ void cf_check paravirt_ctxt_switch_from(struct vcpu *v)
 
 void cf_check paravirt_ctxt_switch_to(struct vcpu *v)
 {
-    root_pgentry_t *root_pgt = this_cpu(root_pgt);
+    const struct domain *d = v->domain;
 
-    if ( root_pgt )
-        root_pgt[root_table_offset(PERDOMAIN_VIRT_START)] =
-            l4e_from_page(v->domain->arch.perdomain_l3_pg,
-                          __PAGE_HYPERVISOR_RW);
+    if ( d->arch.pv.xpti )
+        this_cpu(root_pgt)[root_table_offset(PERDOMAIN_VIRT_START)] =
+            l4e_from_page(d->arch.perdomain_l3_pg, __PAGE_HYPERVISOR_RW);
 
     if ( unlikely(v->arch.dr7 & DR7_ACTIVE_MASK) )
         activate_debugregs(v);
-- 
2.46.0