[PATCH] x86: use default cache size if it cannot be determined [BZ #28784]

[Aurelien Jarno]

In some cases (e.g QEMU, non-Intel/AMD CPU) the cache information can
not be retrieved and the corresponding values are set to 0.

Commit 2d651eb9265d ("x86: Move x86 processor cache info to
cpu_features") changed the behaviour in such case by defining the
__x86_shared_cache_size and __x86_data_cache_size variables to 0 instead
of using the default values. This cause an issue with the i686 SSE2
optimized bzero/routine which assumes that the cache size is at least
128 bytes, and otherwise tries to zero/set the whole address space minus
128 bytes.

Fix that by restoring the original code to only update
__x86_shared_cache_size and __x86_data_cache_size variables if the
corresponding cache sizes are not zero.

Fixes bug 28784
Fixes commit 2d651eb9265d
---
 sysdeps/x86/cacheinfo.h | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/sysdeps/x86/cacheinfo.h b/sysdeps/x86/cacheinfo.h
index 4f91a1e98d..65132a9d19 100644
--- a/sysdeps/x86/cacheinfo.h
+++ b/sysdeps/x86/cacheinfo.h
@@ -61,14 +61,20 @@ init_cacheinfo (void)
   long int data = cpu_features->data_cache_size;
   /* Round data cache size to multiple of 256 bytes.  */
   data = data & ~255L;
-  __x86_data_cache_size_half = data / 2;
-  __x86_data_cache_size = data;
+  if (data > 0)
+    {
+      __x86_data_cache_size_half = data / 2;
+      __x86_data_cache_size = data;
+    }
 
   long int shared = cpu_features->shared_cache_size;
   /* Round shared cache size to multiple of 256 bytes.  */
   shared = shared & ~255L;
-  __x86_shared_cache_size_half = shared / 2;
-  __x86_shared_cache_size = shared;
+  if (shared > 0)
+    {
+      __x86_shared_cache_size_half = shared / 2;
+      __x86_shared_cache_size = shared;
+    }
 
   __x86_shared_non_temporal_threshold
     = cpu_features->non_temporal_threshold;
-- 
2.34.1