[PATCH v4 8/9] x86: introduce GADDR based secondary time area registration alternative

[Jan Beulich <jbeulich@xxxxxxxx>]

The registration by virtual/linear address has downsides: The access is
expensive for HVM/PVH domains. Furthermore for 64-bit PV domains the area
is inaccessible (and hence cannot be updated by Xen) when in guest-user
mode.

Introduce a new vCPU operation allowing to register the secondary time
area by guest-physical address.

An at least theoretical downside to using physically registered areas is
that PV then won't see dirty (and perhaps also accessed) bits set in its
respective page table entries.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
v4: Add xref in public header.
v3: Re-base.
v2: Forge version in force_update_secondary_system_time().

--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1529,6 +1529,15 @@ int arch_vcpu_reset(struct vcpu *v)
     return 0;
 }
 
+static void cf_check
+time_area_populate(void *map, struct vcpu *v)
+{
+    if ( is_pv_vcpu(v) )
+        v->arch.pv.pending_system_time.version = 0;
+
+    force_update_secondary_system_time(v, map);
+}
+
 long do_vcpu_op(int cmd, unsigned int vcpuid, XEN_GUEST_HANDLE_PARAM(void) arg)
 {
     long rc = 0;
@@ -1566,6 +1575,25 @@ long do_vcpu_op(int cmd, unsigned int vc
 
         break;
     }
+
+    case VCPUOP_register_vcpu_time_phys_area:
+    {
+        struct vcpu_register_time_memory_area area;
+
+        rc = -EFAULT;
+        if ( copy_from_guest(&area.addr.p, arg, 1) )
+            break;
+
+        rc = map_guest_area(v, area.addr.p,
+                            sizeof(vcpu_time_info_t),
+                            &v->arch.time_guest_area,
+                            time_area_populate);
+        if ( rc == -ERESTART )
+            rc = hypercall_create_continuation(__HYPERVISOR_vcpu_op, "iih",
+                                               cmd, vcpuid, arg);
+
+        break;
+    }
 
     case VCPUOP_get_physid:
     {
--- a/xen/arch/x86/include/asm/domain.h
+++ b/xen/arch/x86/include/asm/domain.h
@@ -692,6 +692,8 @@ void domain_cpu_policy_changed(struct do
 
 bool update_secondary_system_time(struct vcpu *,
                                   struct vcpu_time_info *);
+void force_update_secondary_system_time(struct vcpu *,
+                                        struct vcpu_time_info *);
 
 void vcpu_show_registers(const struct vcpu *);
 
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1628,6 +1628,16 @@ void force_update_vcpu_system_time(struc
     __update_vcpu_system_time(v, 1);
 }
 
+void force_update_secondary_system_time(struct vcpu *v,
+                                        struct vcpu_time_info *map)
+{
+    struct vcpu_time_info u;
+
+    collect_time_info(v, &u);
+    u.version = -1; /* Compensate for version_update_end(). */
+    write_time_guest_area(map, &u);
+}
+
 static void update_domain_rtc(void)
 {
     struct domain *d;
--- a/xen/arch/x86/x86_64/domain.c
+++ b/xen/arch/x86/x86_64/domain.c
@@ -115,6 +115,7 @@ compat_vcpu_op(int cmd, unsigned int vcp
 
     case VCPUOP_send_nmi:
     case VCPUOP_get_physid:
+    case VCPUOP_register_vcpu_time_phys_area:
         rc = do_vcpu_op(cmd, vcpuid, arg);
         break;
 
--- a/xen/include/public/vcpu.h
+++ b/xen/include/public/vcpu.h
@@ -209,6 +209,9 @@ DEFINE_XEN_GUEST_HANDLE(vcpu_get_physid_
  * segment limit).  It can then apply the normal algorithm to compute
  * system time from the tsc.
  *
+ * New code wants to prefer VCPUOP_register_vcpu_time_phys_area, and only
+ * fall back to the operation here for backwards compatibility.
+ *
  * @extra_arg == pointer to vcpu_register_time_info_memory_area structure.
  */
 #define VCPUOP_register_vcpu_time_memory_area   13
@@ -235,6 +238,7 @@ DEFINE_XEN_GUEST_HANDLE(vcpu_register_ti
  * VMASST_TYPE_runstate_update_flag engaged by the domain.
  */
 #define VCPUOP_register_runstate_phys_area      14
+#define VCPUOP_register_vcpu_time_phys_area     15
 
 #endif /* __XEN_PUBLIC_VCPU_H__ */