[PATCH] ArmPkg/ArmGenericTimerVirtCounterLib: deal with broken generic timers

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

Users of ArmGenericTimerVirtCounterLib may execute under virtualization,
which implies that they may be affected by core errata of the host.

Some implementations of the ARM Generic Timer are affected by errata where
reads of the counter and reads or writes to the timer value may execute
incorrectly when issued around the time the counter is incremented by
the hardware.

Since we can easily work around this without affecting performance too
much, implement an unconditional workaround that compares two subsequent
reads of the counter to ensure the value is correct. Note that the number
for attempts should be limited to avoid breaking platforms such as QEMU
with TCG emulation, since that has been observed never to return the same
value from back to back reads of the counter register.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---

Note that this patch applies on top of the patch 'ArmPkg/ArmLib: remove
indirection layer from timer register accessors' that I send out earlier
today.

 ArmPkg/Library/ArmGenericTimerVirtCounterLib/ArmGenericTimerVirtCounterLib.c | 51 ++++++++++++++++++--
 1 file changed, 48 insertions(+), 3 deletions(-)

diff --git a/ArmPkg/Library/ArmGenericTimerVirtCounterLib/ArmGenericTimerVirtCounterLib.c b/ArmPkg/Library/ArmGenericTimerVirtCounterLib/ArmGenericTimerVirtCounterLib.c
index 69a4ceb62db6..9fe673e8222c 100644
--- a/ArmPkg/Library/ArmGenericTimerVirtCounterLib/ArmGenericTimerVirtCounterLib.c
+++ b/ArmPkg/Library/ArmGenericTimerVirtCounterLib/ArmGenericTimerVirtCounterLib.c
@@ -70,13 +70,36 @@ ArmGenericTimerGetTimerFreq (
   return ArmReadCntFrq ();
 }
 
+//
+// The virtual counter may be used under virtualization on a host that
+// is affected by one of the various errata where reads to the counter
+// register may return incorrect values when the access occurs at the exact
+// time that the counter is incremented by the hardware. This affects the
+// timer as well as the counter.
+// So repeat  the read until we get the same value twice. Unfortunately,
+// platforms such as QEMU with TCG emulation (i.e., non-virtualized) appear
+// never to return the same value twice, so we need to set a retry limit.
+//
+#define MAX_RETRIES   200
+
 UINTN
 EFIAPI
 ArmGenericTimerGetTimerVal (
   VOID
   )
 {
-  return ArmReadCntvTval ();
+  UINTN Result;
+  UINTN Tries;
+
+  Tries = 0;
+  do {
+    //
+    // Keep reading until we see the same value twice in a row. See above.
+    //
+    Result = ArmReadCntvTval ();
+  } while (Result != ArmReadCntvTval () && ++Tries < MAX_RETRIES);
+
+  return Result;
 }
 
 
@@ -86,7 +109,18 @@ ArmGenericTimerSetTimerVal (
   IN   UINTN   Value
   )
 {
-  ArmWriteCntvTval (Value);
+  UINTN CounterVal;
+  UINTN Tries;
+
+  Tries = 0;
+  do {
+    //
+    // Read the counter before and after the write to TVAL, to ensure that
+    // the write to TVAL did not involve a corrupted sample of the counter.
+    //
+    CounterVal = ArmReadCntvCt ();
+    ArmWriteCntvTval (Value);
+  } while (CounterVal != ArmReadCntvCt () && ++Tries < MAX_RETRIES);
 }
 
 UINT64
@@ -95,7 +129,18 @@ ArmGenericTimerGetSystemCount (
   VOID
   )
 {
-  return ArmReadCntvCt ();
+  UINT64 Result;
+  UINTN Tries;
+
+  Tries = 0;
+  do {
+    //
+    // Keep reading until we see the same value twice in a row. See above.
+    //
+    Result = ArmReadCntvCt ();
+  } while (Result != ArmReadCntvCt () && ++Tries < MAX_RETRIES);
+
+  return Result;
 }
 
 UINTN
-- 
2.7.4