x86: support cache-writeback in flush_area_local() et al

The majority of the present callers really aren't after invalidating
cache contents, but only after writeback. Make this available by simply
extending the FLUSH_CACHE handling accordingly. No feature checks are
required here: cache_writeback() falls back to cache_flush() as
necessary, while WBNOINVD degenerates to WBINVD on older hardware.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

xen/arch/x86/flushtlb.c

@@ -232,7 +232,7 @@ unsigned int flush_area_local(const void *va, unsigned int flags)
     if ( flags & FLUSH_HVM_ASID_CORE )
         hvm_flush_guest_tlbs();
 
-    if ( flags & FLUSH_CACHE )
+    if ( flags & (FLUSH_CACHE | FLUSH_CACHE_WRITEBACK) )
     {
         const struct cpuinfo_x86 *c = &current_cpu_data;
         unsigned long sz = 0;
@@ -245,13 +245,16 @@ unsigned int flush_area_local(const void *va, unsigned int flags)
              c->x86_clflush_size && c->x86_cache_size && sz &&
              ((sz >> 10) < c->x86_cache_size) )
         {
-            cache_flush(va, sz);
-            flags &= ~FLUSH_CACHE;
+            if ( flags & FLUSH_CACHE )
+                cache_flush(va, sz);
+            else
+                cache_writeback(va, sz);
+            flags &= ~(FLUSH_CACHE | FLUSH_CACHE_WRITEBACK);
         }
-        else
-        {
+        else if ( flags & FLUSH_CACHE )
             wbinvd();
-        }
+        else
+            wbnoinvd();
     }
 
     if ( flags & FLUSH_ROOT_PGTBL )

xen/arch/x86/include/asm/flushtlb.h

@@ -135,6 +135,8 @@ void switch_cr3_cr4(unsigned long cr3, unsigned long cr4);
 #else
 # define FLUSH_NO_ASSIST 0
 #endif
+ /* Write back data cache contents */
+#define FLUSH_CACHE_WRITEBACK  0x10000
 
 /* Flush local TLBs/caches. */
 unsigned int flush_area_local(const void *va, unsigned int flags);
@@ -195,7 +197,11 @@ static inline int clean_and_invalidate_dcache_va_range(const void *p,
 }
 static inline int clean_dcache_va_range(const void *p, unsigned long size)
 {
-    return clean_and_invalidate_dcache_va_range(p, size);
+    unsigned int order = get_order_from_bytes(size);
+
+    /* sub-page granularity support needs to be added if necessary */
+    flush_area_local(p, FLUSH_CACHE_WRITEBACK | FLUSH_ORDER(order));
+    return 0;
 }
 
 unsigned int guest_flush_tlb_flags(const struct domain *d);