From 603ad2d6ae7c0b46815123a59a2b8c8169711c0e Mon Sep 17 00:00:00 2001
From: Gerd Hoffmann <kraxel@redhat.com>
Date: Fri, 1 Mar 2024 11:23:58 +0100
Subject: OvmfPkg/PlatformInitLib: add support for GuestPhysBits

Add support for GuestPhysBits (cpuid 0x80000008, eax, bits 23:16).

GuestPhysBits is a field which can be set by the hypervisor to inform
the guest about the /usable/ physical address space bits.  This can be
smaller than the PhysBits of the CPU, for example because of nested
paging limitations.

OVMF will read GuestPhysBits, log the value, in case it is set use it
as upper limit.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 OvmfPkg/Library/PlatformInitLib/MemDetect.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

(limited to 'OvmfPkg')

diff --git a/OvmfPkg/Library/PlatformInitLib/MemDetect.c b/OvmfPkg/Library/PlatformInitLib/MemDetect.c
index e64c0ee..f531b98 100644
--- a/OvmfPkg/Library/PlatformInitLib/MemDetect.c
+++ b/OvmfPkg/Library/PlatformInitLib/MemDetect.c
@@ -633,6 +633,7 @@ PlatformAddressWidthFromCpuid (
 {
   UINT32    RegEax, RegEbx, RegEcx, RegEdx, Max;
   UINT8     PhysBits;
+  UINT8     GuestPhysBits;
   CHAR8     Signature[13];
   IA32_CR4  Cr4;
   BOOLEAN   Valid         = FALSE;
@@ -655,13 +656,17 @@ PlatformAddressWidthFromCpuid (
 
   if (Max >= 0x80000008) {
     AsmCpuid (0x80000008, &RegEax, NULL, NULL, NULL);
-    PhysBits = (UINT8)RegEax;
+    PhysBits      = (UINT8)RegEax;
+    GuestPhysBits = (UINT8)(RegEax >> 16);
   } else {
-    PhysBits = 36;
+    PhysBits      = 36;
+    GuestPhysBits = 0;
   }
 
   if (!QemuQuirk) {
     Valid = TRUE;
+  } else if (GuestPhysBits) {
+    Valid = TRUE;
   } else if (PhysBits >= 41) {
     Valid = TRUE;
   } else if (AsciiStrCmp (Signature, "GenuineIntel") == 0) {
@@ -678,15 +683,21 @@ PlatformAddressWidthFromCpuid (
 
   DEBUG ((
     DEBUG_INFO,
-    "%a: Signature: '%a', PhysBits: %d, QemuQuirk: %a, la57: %a, Valid: %a\n",
+    "%a: Signature: '%a', PhysBits: %d, GuestPhysBits: %d, QemuQuirk: %a, la57: %a, Valid: %a\n",
     __func__,
     Signature,
     PhysBits,
+    GuestPhysBits,
     QemuQuirk ? "On" : "Off",
     Cr4.Bits.LA57 ? "On" : "Off",
     Valid ? "Yes" : "No"
     ));
 
+  if (GuestPhysBits && (PhysBits > GuestPhysBits)) {
+    DEBUG ((DEBUG_INFO, "%a: limit PhysBits to %d (GuestPhysBits)\n", __func__, GuestPhysBits));
+    PhysBits = GuestPhysBits;
+  }
+
   if (Valid) {
     /*
      * Due to the sign extension we can use only the lower half of the
-- 
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