[PATCH 08/12] x86/paging: deal with log-dirty stats overflow

[Jan Beulich <jbeulich@xxxxxxxx>]

While the precise values are unlikely of interest once they exceed 4
billion (allowing us to leave alone the domctl struct), we still
shouldn't wrap or truncate the actual values. It is in particular
problematic if the truncated values were zero (causing libxenguest to
skip an iteration altogether) or a very small value (leading to
premature exiting of the pre-copy phase).

Change the internal fields to unsigned long, and suitably saturate for
copying to guest context.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/mm/paging.c
+++ b/xen/arch/x86/mm/paging.c
@@ -446,14 +446,16 @@ static int paging_log_dirty_op(struct do
 
     clean = (sc->op == XEN_DOMCTL_SHADOW_OP_CLEAN);
 
-    PAGING_DEBUG(LOGDIRTY, "log-dirty %s: dom %u faults=%u dirty=%u\n",
+    PAGING_DEBUG(LOGDIRTY, "log-dirty %s: dom %u faults=%lu dirty=%lu\n",
                  (clean) ? "clean" : "peek",
                  d->domain_id,
                  d->arch.paging.log_dirty.fault_count,
                  d->arch.paging.log_dirty.dirty_count);
 
-    sc->stats.fault_count = d->arch.paging.log_dirty.fault_count;
-    sc->stats.dirty_count = d->arch.paging.log_dirty.dirty_count;
+    sc->stats.fault_count = min(d->arch.paging.log_dirty.fault_count,
+                                UINT32_MAX + 0UL);
+    sc->stats.dirty_count = min(d->arch.paging.log_dirty.dirty_count,
+                                UINT32_MAX + 0UL);
 
     if ( guest_handle_is_null(sc->dirty_bitmap) )
         /* caller may have wanted just to clean the state or access stats. */
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -190,8 +190,8 @@ struct log_dirty_domain {
     unsigned int   failed_allocs;
 
     /* log-dirty mode stats */
-    unsigned int   fault_count;
-    unsigned int   dirty_count;
+    unsigned long  fault_count;
+    unsigned long  dirty_count;
 
     /* functions which are paging mode specific */
     const struct log_dirty_ops {