1 files changed, 40 insertions, 0 deletions
diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
index 5aabe9d7b5..565383ee26 100644
--- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
+++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
@@ -10,6 +10,9 @@
 **/
 
 #include <Library/UefiRuntimeLib.h>
+#include <Library/MemEncryptSevLib.h>
+#include <Library/VmgExitLib.h>
+#include <Register/Amd/Msr.h>
 
 #include "QemuFlash.h"
 
@@ -32,3 +35,40 @@ QemuFlashBeforeProbe (
   // Do nothing
   //
 }
+
+/**
+  Write to QEMU Flash
+
+  @param[in] Ptr    Pointer to the location to write.
+  @param[in] Value  The value to write.
+
+**/
+VOID
+QemuFlashPtrWrite (
+  IN        volatile UINT8    *Ptr,
+  IN        UINT8             Value
+  )
+{
+  if (MemEncryptSevEsIsEnabled ()) {
+    MSR_SEV_ES_GHCB_REGISTER  Msr;
+    GHCB                      *Ghcb;
+
+    Msr.GhcbPhysicalAddress = AsmReadMsr64 (MSR_SEV_ES_GHCB);
+    Ghcb = Msr.Ghcb;
+
+    //
+    // Writing to flash is emulated by the hypervisor through the use of write
+    // protection. This won't work for an SEV-ES guest because the write won't
+    // be recognized as a true MMIO write, which would result in the required
+    // #VC exception. Instead, use the the VMGEXIT MMIO write support directly
+    // to perform the update.
+    //
+    VmgInit (Ghcb);
+    Ghcb->SharedBuffer[0] = Value;
+    Ghcb->SaveArea.SwScratch = (UINT64) (UINTN) Ghcb->SharedBuffer;
+    VmgExit (Ghcb, SVM_EXIT_MMIO_WRITE, (UINT64) (UINTN) Ptr, 1);
+    VmgDone (Ghcb);
+  } else {
+    *Ptr = Value;
+  }
+}