x86/pv: fix MMUEXT_FLUSH_CACHE to flush all pCPU caches

The implementation of MMUEXT_FLUSH_CACHE is bogus, as it doesn't account to
flush the cache of any previous pCPU where the current vCPU might have run,
and hence is likely to not work as expected.

Fix this by resorting to use the same logic as MMUEXT_FLUSH_CACHE_GLOBAL,
which will be correct in all cases.

Adjust MMUEXT_FLUSH_CACHE_GLOBAL return code in case cache flush is not
permitted for the domain to use -EACCES instead of -EINVAL, as that's more
accurate and also matches the error code used by MMUEXT_FLUSH_CACHE.

Fixes: 8e90e37e6d ("Fix WBINVD by adding a new hypercall.")
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
master commit: bbaea9878b
master date: 2025-05-13 19:43:48 +0200

xen/arch/x86/mm.c

@@ -3811,14 +3811,11 @@ long do_mmuext_op(
             break;
 
         case MMUEXT_FLUSH_CACHE:
-            if ( unlikely(currd != pg_owner) )
-                rc = -EPERM;
-            else if ( unlikely(!cache_flush_permitted(currd)) )
-                rc = -EACCES;
-            else
-                wbinvd();
-            break;
-
+            /*
+             * Dirty pCPU caches where the current vCPU has been scheduled are
+             * not tracked, and hence we need to resort to a global cache
+             * flush for correctness.
+             */
         case MMUEXT_FLUSH_CACHE_GLOBAL:
             if ( unlikely(currd != pg_owner) )
                 rc = -EPERM;
@@ -3835,7 +3832,7 @@ long do_mmuext_op(
                 flush_mask(mask, FLUSH_CACHE);
             }
             else
-                rc = -EINVAL;
+                rc = -EACCES;
             break;
 
         case MMUEXT_SET_LDT: