xen/arm: ffa: use ACCESS_ONCE()

Replace read_atomic() with ACCESS_ONCE() to match the intended use, that
is, to prevent the compiler from (via optimization) reading shared
memory more than once.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

xen/arch/arm/tee/ffa_shm.c

@@ -7,6 +7,7 @@
 #include <xen/sizes.h>
 #include <xen/types.h>
 #include <xen/mm.h>
+#include <xen/lib.h>
 #include <xen/list.h>
 #include <xen/spinlock.h>
 
@@ -171,8 +172,8 @@ static int get_shm_pages(struct domain *d, struct ffa_shm_mem *shm,
 
     for ( n = 0; n < range_count; n++ )
     {
-        page_count = read_atomic(&range[n].page_count);
-        addr = read_atomic(&range[n].address);
+        page_count = ACCESS_ONCE(range[n].page_count);
+        addr = ACCESS_ONCE(range[n].address);
         for ( m = 0; m < page_count; m++ )
         {
             if ( pg_idx >= shm->page_count )
@@ -527,13 +528,13 @@ void ffa_handle_mem_share(struct cpu_user_regs *regs)
         goto out_unlock;
 
     mem_access = ctx->tx + trans.mem_access_offs;
-    if ( read_atomic(&mem_access->access_perm.perm) != FFA_MEM_ACC_RW )
+    if ( ACCESS_ONCE(mem_access->access_perm.perm) != FFA_MEM_ACC_RW )
     {
         ret = FFA_RET_NOT_SUPPORTED;
         goto out_unlock;
     }
 
-    region_offs = read_atomic(&mem_access->region_offs);
+    region_offs = ACCESS_ONCE(mem_access->region_offs);
     if ( sizeof(*region_descr) + region_offs > frag_len )
     {
         ret = FFA_RET_NOT_SUPPORTED;
@@ -541,8 +542,8 @@ void ffa_handle_mem_share(struct cpu_user_regs *regs)
     }
 
     region_descr = ctx->tx + region_offs;
-    range_count = read_atomic(&region_descr->address_range_count);
-    page_count = read_atomic(&region_descr->total_page_count);
+    range_count = ACCESS_ONCE(region_descr->address_range_count);
+    page_count = ACCESS_ONCE(region_descr->total_page_count);
 
     if ( !page_count )
     {
@@ -557,7 +558,7 @@ void ffa_handle_mem_share(struct cpu_user_regs *regs)
         goto out_unlock;
     }
     shm->sender_id = trans.sender_id;
-    shm->ep_id = read_atomic(&mem_access->access_perm.endpoint_id);
+    shm->ep_id = ACCESS_ONCE(mem_access->access_perm.endpoint_id);
 
     /*
      * Check that the Composite memory region descriptor fits.