Re: [PATCH v2 2/4] OvmfPkg/PlatformInitLib: detect physical address space

["Gerd Hoffmann" <kraxel@redhat.com>]

  Hi,

> With edk2-stable202205 or when reverting the commit, my VM boots with
> only 512 MiB of assigned memory. With the commit, with 512 MiB it cannot
> even initialize the display, with 750 MiB it runs into a different error
> during Linux boot, and with 900 MiB it boots fine again.
> 
> So it seems that having a larger PhysMemAddressWidth requires more
> memory? Is this expected and does running VMs with this version simply
> require more memory now? Is there a way to avoid that?

Might be more memory being needed for page tables.  Making sure gigabyte
pages are enabled (-cpu $name,pdpe1gb=on), so edk2 can use them for the
mmio window identity mapping.

Alternatively try change the cap:

------------------------------------------------------------
diff --git a/OvmfPkg/Library/PlatformInitLib/MemDetect.c b/OvmfPkg/Library/PlatformInitLib/MemDetect.c
index 38cece9173e8..76df6fd020bf 100644
--- a/OvmfPkg/Library/PlatformInitLib/MemDetect.c
+++ b/OvmfPkg/Library/PlatformInitLib/MemDetect.c
@@ -669,9 +669,9 @@ PlatformAddressWidthFromCpuid (
       PhysBits = 47;
     }
 
-    if (!Page1GSupport && (PhysBits > 40)) {
-      DEBUG ((DEBUG_INFO, "%a: limit PhysBits to 40 (no 1G pages available)\n", __func__));
-      PhysBits = 40;
+    if (!Page1GSupport && (PhysBits > 38)) {
+      DEBUG ((DEBUG_INFO, "%a: limit PhysBits to 38 (no 1G pages available)\n", __func__));
+      PhysBits = 38;
     }
 
     PlatformInfoHob->PhysMemAddressWidth = PhysBits;
------------------------------------------------------------

HTH,
  Gerd