[RFC,v2,2/8] domain: update GADDR based runstate guest area

Commit Message

Jan Beulich Jan. 23, 2023, 2:53 p.m. UTC

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.

Note that updating of the area will be done exclusively following the
model enabled by VMASST_TYPE_runstate_update_flag for virtual-address
based registered areas.

Note further that pages aren't marked dirty when written to (matching
the handling of space mapped by map_vcpu_info()), on the basis that the
registrations are lost anyway across migration (or would need re-
populating at the target for transparent 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).

Signed-off-by: Jan Beulich <[e-mail]>
---
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).
---
v2: Drop VM-assist conditionals.

Patch

--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -1616,14 +1616,53 @@  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;
+#ifdef CONFIG_COMPAT
+        struct compat_vcpu_runstate_info *cmap = NULL;
+
+        if ( v->runstate_guest_area_compat )
+            cmap = (void *)map;
+#endif
+
+        /*
+         * NB: No VM_ASSIST(v->domain, runstate_update_flag) check here.
+         *     Always using that updating model.
+         */
+#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;
+
+        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