OvmfPkg/VirtHstiDxe: add varstore flash check

Detects qemu config issue: vars pflash is not in secure mode (write
access restricted to smm).  Applies to Q35 with SMM only.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Konstantin Kostiuk <kkostiuk@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jiewen Yao <Jiewen.yao@intel.com>

OvmfPkg/VirtHstiDxe/Flash.c

@@ -0,0 +1,90 @@
+/** @file
+
+SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+#include <Library/BaseLib.h>
+#include <Library/DebugLib.h>
+
+#include "VirtHstiDxe.h"
+
+#define WRITE_BYTE_CMD           0x10
+#define BLOCK_ERASE_CMD          0x20
+#define CLEAR_STATUS_CMD         0x50
+#define READ_STATUS_CMD          0x70
+#define READ_DEVID_CMD           0x90
+#define BLOCK_ERASE_CONFIRM_CMD  0xd0
+#define READ_ARRAY_CMD           0xff
+#define CLEARED_ARRAY_STATUS     0x00
+
+/* based on QemuFlashDetected (QemuFlashFvbServicesRuntimeDxe) */
+UINT32
+VirtHstiQemuFirmwareFlashCheck (
+  UINT32  Address
+  )
+{
+  volatile UINT8  *Ptr;
+
+  UINTN  Offset;
+  UINT8  OriginalUint8;
+  UINT8  ProbeUint8;
+
+  for (Offset = 0; Offset < EFI_PAGE_SIZE; Offset++) {
+    Ptr        = (UINT8 *)(UINTN)(Address + Offset);
+    ProbeUint8 = *Ptr;
+    if ((ProbeUint8 != CLEAR_STATUS_CMD) &&
+        (ProbeUint8 != READ_STATUS_CMD) &&
+        (ProbeUint8 != CLEARED_ARRAY_STATUS))
+    {
+      break;
+    }
+  }
+
+  if (Offset >= EFI_PAGE_SIZE) {
+    DEBUG ((DEBUG_INFO, "%a: check failed\n", __func__));
+    return QEMU_FIRMWARE_FLASH_UNKNOWN;
+  }
+
+  OriginalUint8 = *Ptr;
+  *Ptr          = CLEAR_STATUS_CMD;
+  ProbeUint8    = *Ptr;
+  if ((OriginalUint8 != CLEAR_STATUS_CMD) &&
+      (ProbeUint8 == CLEAR_STATUS_CMD))
+  {
+    *Ptr = OriginalUint8;
+    DEBUG ((DEBUG_INFO, "%a: %p behaves as RAM\n", __func__, Ptr));
+    return QEMU_FIRMWARE_FLASH_IS_RAM;
+  }
+
+  *Ptr       = READ_STATUS_CMD;
+  ProbeUint8 = *Ptr;
+  if (ProbeUint8 == OriginalUint8) {
+    DEBUG ((DEBUG_INFO, "%a: %p behaves as ROM\n", __func__, Ptr));
+    return QEMU_FIRMWARE_FLASH_IS_ROM;
+  }
+
+  if (ProbeUint8 == READ_STATUS_CMD) {
+    *Ptr = OriginalUint8;
+    DEBUG ((DEBUG_INFO, "%a: %p behaves as RAM\n", __func__, Ptr));
+    return QEMU_FIRMWARE_FLASH_IS_RAM;
+  }
+
+  if (ProbeUint8 == CLEARED_ARRAY_STATUS) {
+    *Ptr       = WRITE_BYTE_CMD;
+    *Ptr       = OriginalUint8;
+    *Ptr       = READ_STATUS_CMD;
+    ProbeUint8 = *Ptr;
+    *Ptr       = READ_ARRAY_CMD;
+    if (ProbeUint8 & 0x10 /* programming error */) {
+      DEBUG ((DEBUG_INFO, "%a: %p behaves as FLASH, write-protected\n", __func__, Ptr));
+      return QEMU_FIRMWARE_FLASH_READ_ONLY;
+    } else {
+      DEBUG ((DEBUG_INFO, "%a: %p behaves as FLASH, writable\n", __func__, Ptr));
+      return QEMU_FIRMWARE_FLASH_WRITABLE;
+    }
+  }
+
+  DEBUG ((DEBUG_INFO, "%a: check failed\n", __func__));
+  return QEMU_FIRMWARE_FLASH_UNKNOWN;
+}