Prefer the e820 map provided via qemu firmware config interface
for memory detection.  Use rtc cmos only as fallback, which should
be rarely needed these days as qemu supports etc/e820 since 2013.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3593
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
---
 OvmfPkg/PlatformPei/MemDetect.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/OvmfPkg/PlatformPei/MemDetect.c b/OvmfPkg/PlatformPei/MemDetect.c
index 500c1d4d5231..1d942b12d519 100644
--- a/OvmfPkg/PlatformPei/MemDetect.c
+++ b/OvmfPkg/PlatformPei/MemDetect.c
@@ -293,9 +293,16 @@ GetSystemMemorySizeBelow4gb (
   VOID
   )
 {
+  EFI_STATUS Status;
+  UINT64 LowerMemorySize = 0;
   UINT8 Cmos0x34;
   UINT8 Cmos0x35;
 
+  Status = ScanOrAdd64BitE820Ram (FALSE, &LowerMemorySize, NULL);
+  if (Status == EFI_SUCCESS && LowerMemorySize > 0) {
+    return (UINT32)LowerMemorySize;
+  }
+
   //
   // CMOS 0x34/0x35 specifies the system memory above 16 MB.
   // * CMOS(0x35) is the high byte
@@ -722,7 +729,6 @@ QemuInitializeRam (
   // Determine total memory size available
   //
   LowerMemorySize = GetSystemMemorySizeBelow4gb ();
-  UpperMemorySize = GetSystemMemorySizeAbove4gb ();
 
   if (mBootMode == BOOT_ON_S3_RESUME) {
     //
@@ -769,8 +775,11 @@ QemuInitializeRam (
     // memory size read from the CMOS.
     //
     Status = ScanOrAdd64BitE820Ram (TRUE, NULL, NULL);
-    if (EFI_ERROR (Status) && UpperMemorySize != 0) {
-      AddMemoryBaseSizeHob (BASE_4GB, UpperMemorySize);
+    if (EFI_ERROR (Status)) {
+      UpperMemorySize = GetSystemMemorySizeAbove4gb ();
+      if (UpperMemorySize != 0) {
+        AddMemoryBaseSizeHob (BASE_4GB, UpperMemorySize);
+      }
     }
   }
 
-- 
2.31.1



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