x86/vtx: Allow runtime modification of the exec-sp setting

See patch for details.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>

docs/misc/xen-command-line.pandoc

@@ -959,6 +959,21 @@ introduced with the Nehalem architecture.
     If HVM guest kernels are trusted not to mount a DoS against the system,
     this option can enabled to regain performance.
 
+    This boolean may be modified at runtime using `xl set-parameters
+    ept=[no-]exec-sp` to switch between fast and secure.
+
+    *   When switching from secure to fast, preexisting HVM domains will run
+        at their current performance until they are rebooted; new domains will
+        run without any overhead.
+
+    *   When switching from fast to secure, all HVM domains will immediately
+        suffer a performance penalty.
+
+    **Warning: No guarantee is made that this runtime option will be retained
+      indefinitely, or that it will retain this exact behaviour.  It is
+      intended as an emergency option for people who first chose fast, then
+      change their minds to secure, and wish not to reboot.**
+
 ### extra_guest_irqs
 > `= [<domU number>][,<dom0 number>]`
 

xen/arch/x86/hvm/vmx/vmcs.c

@@ -95,6 +95,41 @@ static int __init parse_ept_param(const char *s)
 }
 custom_param("ept", parse_ept_param);
 
+static int parse_ept_param_runtime(const char *s)
+{
+    int val;
+
+    if ( !cpu_has_vmx_ept || !hvm_funcs.hap_supported ||
+         !(hvm_funcs.hap_capabilities &
+           (HVM_HAP_SUPERPAGE_2MB | HVM_HAP_SUPERPAGE_1GB)) )
+    {
+        printk("VMX: EPT not available, or not in use - ignoring\n");
+        return 0;
+    }
+
+    if ( (val = parse_boolean("exec-sp", s, NULL)) < 0 )
+        return -EINVAL;
+
+    if ( val != opt_ept_exec_sp )
+    {
+        struct domain *d;
+
+        opt_ept_exec_sp = val;
+
+        rcu_read_lock(&domlist_read_lock);
+        for_each_domain ( d )
+            if ( paging_mode_hap(d) )
+                p2m_change_entry_type_global(d, p2m_ram_rw, p2m_ram_rw);
+        rcu_read_unlock(&domlist_read_lock);
+    }
+
+    printk("VMX: EPT executable superpages %sabled\n",
+           val ? "en" : "dis");
+
+    return 0;
+}
+custom_runtime_only_param("ept", parse_ept_param_runtime);
+
 /* Dynamic (run-time adjusted) execution control flags. */
 u32 vmx_pin_based_exec_control __read_mostly;
 u32 vmx_cpu_based_exec_control __read_mostly;

xen/arch/x86/mm/p2m.c

@@ -289,15 +289,20 @@ static void change_entry_type_global(struct p2m_domain *p2m,
                                      p2m_type_t ot, p2m_type_t nt)
 {
     p2m->change_entry_type_global(p2m, ot, nt);
-    p2m->global_logdirty = (nt == p2m_ram_logdirty);
+    /* Don't allow 'recalculate' operations to change the logdirty state. */
+    if ( ot != nt )
+        p2m->global_logdirty = (nt == p2m_ram_logdirty);
 }
 
+/*
+ * May be called with ot = nt = p2m_ram_rw for its side effect of
+ * recalculating all PTEs in the p2m.
+ */
 void p2m_change_entry_type_global(struct domain *d,
                                   p2m_type_t ot, p2m_type_t nt)
 {
     struct p2m_domain *hostp2m = p2m_get_hostp2m(d);
 
-    ASSERT(ot != nt);
     ASSERT(p2m_is_changeable(ot) && p2m_is_changeable(nt));
 
     p2m_lock(hostp2m);