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.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/mm/shadow/multi.c

@@ -2597,14 +2597,7 @@ static int cf_check sh_page_fault(
 
     /* Ignore attempts to write to read-only memory. */
     if ( p2m_is_readonly(p2mt) && (ft == ft_demand_write) )
-    {
-        static unsigned long lastpage;
-        if ( xchg(&lastpage, va & PAGE_MASK) != (va & PAGE_MASK) )
-            gdprintk(XENLOG_DEBUG, "guest attempted write to read-only memory"
-                     " page. va page=%#lx, mfn=%#lx\n",
-                     va & PAGE_MASK, mfn_x(gmfn));
         goto emulate_readonly; /* skip over the instruction */
-    }
 
     /* In HVM guests, we force CR0.WP always to be set, so that the
      * pagetables are always write-protected.  If the guest thinks