[PATCH RFC 04/10] domain: update GADDR based runstate guest area

[Jan Beulich <jbeulich@xxxxxxxx>]

Before adding a new vCPU operation to register the runstate area by
guest-physical address, add code to actually keep such areas up-to-date.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
RFC: Pages aren't marked dirty when written to (matching the handling of
     space mapped by map_vcpu_info() afaict), on the basis that the
     registrations are lost anyway across migration. Plus the contents
     of the areas in question have to be deemed volatile in the first
     place (so saving a "most recent" value is pretty meaningless even
     for e.g. snapshotting).

RFC: Can we perhaps avoid the VM-assist conditionals, assuming the more
     modern behavior to apply uniformly for gaddr-based registrations?

RFC: HVM guests (on x86) can change bitness and hence layout (and size!
     and alignment) of the runstate area. I don't think it is an option
     to require 32-bit code to pass a range such that even the 64-bit
     layout wouldn't cross a page boundary (and be suitably aligned). I
     also don't see any other good solution, so for now a crude approach
     with an extra boolean is used (using has_32bit_shinfo() isn't race
     free and could hence lead to overrunning the mapped space).

--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -1599,14 +1599,55 @@ bool update_runstate_area(struct vcpu *v
     struct guest_memory_policy policy = { };
     void __user *guest_handle = NULL;
     struct vcpu_runstate_info runstate;
+    struct vcpu_runstate_info *map = v->runstate_guest_area.map;
+
+    memcpy(&runstate, &v->runstate, sizeof(runstate));
+
+    if ( map )
+    {
+        uint64_t *pset = NULL;
+#ifdef CONFIG_COMPAT
+        struct compat_vcpu_runstate_info *cmap = NULL;
+
+        if ( v->runstate_guest_area_compat )
+            cmap = (void *)map;
+#endif
+
+        if ( VM_ASSIST(v->domain, runstate_update_flag) )
+        {
+#ifdef CONFIG_COMPAT
+            if ( cmap )
+                pset = &cmap->state_entry_time;
+            else
+#endif
+                pset = &map->state_entry_time;
+            runstate.state_entry_time |= XEN_RUNSTATE_UPDATE;
+            write_atomic(pset, runstate.state_entry_time);
+            smp_wmb();
+        }
+
+#ifdef CONFIG_COMPAT
+        if ( cmap )
+            XLAT_vcpu_runstate_info(cmap, &runstate);
+        else
+#endif
+            *map = runstate;
+
+        if ( pset )
+        {
+            smp_wmb();
+            runstate.state_entry_time &= ~XEN_RUNSTATE_UPDATE;
+            write_atomic(pset, runstate.state_entry_time);
+        }
+
+        return true;
+    }
 
     if ( guest_handle_is_null(runstate_guest(v)) )
         return true;
 
     update_guest_memory_policy(v, &policy);
 
-    memcpy(&runstate, &v->runstate, sizeof(runstate));
-
     if ( VM_ASSIST(v->domain, runstate_update_flag) )
     {
 #ifdef CONFIG_COMPAT
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -231,6 +231,8 @@ struct vcpu
 #ifdef CONFIG_COMPAT
     /* A hypercall is using the compat ABI? */
     bool             hcall_compat;
+    /* Physical runstate area registered via compat ABI? */
+    bool             runstate_guest_area_compat;
 #endif
 
 #ifdef CONFIG_IOREQ_SERVER