crossystem: cache vbnv contents to avoid lengthy reads

Storing nvram in SPI Flash becomes more and more popular. Retrieving
it takes quite a while due to various flashrom issues. While flashrom
still needs to be improved to minimize its running time, a good speed
up can be achieved by caching the nvram contents in crossystem.

The cache is invalidated each time nvram is written (this could be
optimized by updating the local copy, but probably is not worth the
extra effort).

BRANCH=storm
BUG=chrome-os-partner:33592
TEST=crossystem runs much faster now:
  localhost var # time /var/tmp/crossystem
  . . .
  real    0m1.669s
  user    0m0.790s
  sys     0m0.170s
  localhost var #

Change-Id: Ie4a483efc189257ff58c92bdc39871b917c89727
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/228655
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Reviewed-by: David Hendricks <dhendrix@chromium.org>

1 files changed, 11 insertions, 6 deletions

diff --git a/host/lib/crossystem.c b/host/lib/crossystem.c
index b28fd877..43dcc2a4 100644
--- a/host/lib/crossystem.c
+++ b/host/lib/crossystem.c
@@ -80,20 +80,24 @@ int FwidStartsWith(const char *start) {
   return 0 == strncmp(fwid, start, strlen(start));
 }
 
+static int vnc_read;
 
 int VbGetNvStorage(VbNvParam param) {
-  VbNvContext vnc;
   uint32_t value;
   int retval;
+  static VbNvContext cached_vnc;
 
   /* TODO: locking around NV access */
+  if (!vnc_read) {
+    if (0 != VbReadNvStorage(&cached_vnc))
+      return -1;
+    vnc_read = 1;
+  }
 
-  if (0 != VbReadNvStorage(&vnc))
+  if (0 != VbNvSetup(&cached_vnc))
     return -1;
-  if (0 != VbNvSetup(&vnc))
-    return -1;
-  retval = VbNvGet(&vnc, param, &value);
-  if (0 != VbNvTeardown(&vnc))
+  retval = VbNvGet(&cached_vnc, param, &value);
+  if (0 != VbNvTeardown(&cached_vnc))
     return -1;
   if (0 != retval)
     return -1;
@@ -123,6 +127,7 @@ int VbSetNvStorage(VbNvParam param, int value) {
     goto VbSetNvCleanup;
 
   if (vnc.raw_changed) {
+    vnc_read = 0;
     if (0 != VbWriteNvStorage(&vnc))
       goto VbSetNvCleanup;
   }