[PATCH] x86/HVM: purge dubious lastpage diagnostic

[Jan Beulich <jbeulich@xxxxxxxx>]

Quoting b5d8b03db136 ("x86/shadow: Drop dubious lastpage diagnostic"):

"This is a global variable (actually 3, one per GUEST_PAGING_LEVEL), operated
 on using atomics only (with no regard to what else shares the same cacheline),
 which emits a diagnostic (in debug builds only) without changing any program
 behaviour.

 It is presumably left-over debugging, as it interlinks the behaviour of all
 vCPUs in chronological order.  Based on the read-only p2m types, this
 diagnostic can be tripped by entirely legitimate guest behaviour."

All the same applies here (it's only a single variable of course).

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

--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -3354,16 +3354,7 @@ static enum hvm_translation_result __hvm
                 memcpy(buf, p, count);
                 hvmemul_write_cache(v, gfn_to_gaddr(gfn) | pgoff, buf, count);
             }
-            else if ( p2m_is_discard_write(p2mt) )
-            {
-                static unsigned long lastpage;
-
-                if ( xchg(&lastpage, gfn_x(gfn)) != gfn_x(gfn) )
-                    dprintk(XENLOG_G_DEBUG,
-                            "%pv attempted write to read-only gfn %#lx 
(mfn=%#"PRI_mfn")\n",
-                            v, gfn_x(gfn), mfn_x(page_to_mfn(page)));
-            }
-            else
+            else if ( !p2m_is_discard_write(p2mt) )
             {
                 if ( buf )
                     memcpy(p, buf, count);